Genetics: Mendel and Beyond

Transcription

1 10 Genetic: Mendel and Beyond In the Middle Eatern deert 1,800 year ago, the rai faced a dilemma. A Jewih woman had given irth to a on. A required y the law et down y God commandment to Araham almot 2,000 year previouly and later reiterated y Moe, the mother rought her 8-day-old on to the rai for ritual penile circumciion. The rai knew that the woman two previou on had led to death when their forekin were cut. Yet the ilical commandment remained: Unle he wa circumcied, the oy could not e counted among thoe with whom God had made Hi olemn covenant. After conultation with other rai, it wa decided to exempt thi, the third on. Almot a thouand year later, in the twelfth century, the phyician and ilical commentator Moe Maimonide reviewed thi and numerou other cae in the rainical literature and tated that in uch intance the third on hould not e circumcied. Furthermore, the exemption hould apply whether the mother on wa from her firt huand or from her econd huand. The leeding diorder, he reaoned, wa clearly carried y the mother and paed on to her on. Knowing nothing of our modern viion of genetic, thee rai linked a human dieae (which we now know a hemophilia A) to a pattern of inheritance (which we know a ex linkage). Only in the pat few decade have the precie iochemical nature of hemophilia A and it genetic determination een worked out. How do we account for, and predict, uch pattern of inheritance? In thi chapter, we will dicu how the unit of inheritance, called gene, are tranmitted from generation to generation, and we will how how many of the rule that govern genetic can e explained y the ehavior of chromoome during meioi. We will alo decrie the interaction of gene with one another and with the environment, and we will examine the conequence of the fact that gene occupy pecific poition on chromoome. An Ancient Ritual A male infant undergoe ritual circumciion in accordance with Jewih law. on of Jewih mother who carry the gene for hemophilia may e exempted from the ritual. The Foundation of Genetic Much of the early tudy of iological inheritance wa done with plant and animal of economic importance. Record how that people were delierately croreeding date palm tree and hore a early a 5,000 year ago. By the early nineteenth century, plant reed-

2 188 CHAPTER TEN ing wa widepread, epecially with ornamental flower uch a tulip. Half a century later, Gregor Mendel ued the exiting knowledge of plant reproduction to deign and conduct experiment on inheritance. Although hi pulihed reult were neglected y cientit for more than 30 year, they ultimately ecame the foundation for the cience of genetic. Plant reeder howed that oth parent contriute equally to inheritance Plant are good experimental uject for the tudy of genetic. Many plant are eaily grown in large quantitie, produce large numer of offpring (in the form of eed), and have relatively hort generation time. In mot plant pecie, the ame individual have oth male and female reproductive organ, permitting each plant to reproduce a a male, a a female, or a oth. Bet of all, it i often eay to control which individual mate (Figure 10.1). ome dicoverie that Mendel found ueful in hi tudie had een made in the late eighteenth century y a German otanit, Joef Gottlie Kölreuter. He had tudied the offpring of reciprocal croe, in which plant are croed (mated with each other) in oppoite direction. For example, in one cro, male that have white flower are mated with female that have red flower, while in a complementary cro, red-flowered male and white-flowered female are mated. In Kölreuter tudie, uch reciprocal croe alway gave identical reult, howing that oth parent contriuted equally to the offpring. Although the concept of equal parental contriution wa an important dicovery, the nature of what exactly the parent were contriuting to their offpring the unit of inheritance remained unknown. Law of inheritance propoed at the time favored the concept of lending. If a plant that had one form of a characteritic (ay, red flower) wa croed with one that had a different form of that characteritic (lue flower), the offpring would e a lended comination of the two parent (purple flower). According to the lending theory, it wa thought that once heritale element were comined, they could not e eparated again (like ink of different color mixed together). The red and lue genetic determinant were thought to e forever lended into the new purple one. Then, aout a century after Kölreuter completed hi work, Mendel egan hi. Anatomy of a pea flower (hown in long ection) Pea flower cro-pollination Parent plant 2 Thi cro-pollination produce eed that are allowed to grow into new plant. REEARCH METHOD Pollen 1 Uing a ruh, pollen i tranferred from anther of a purple flower to the tigma of a white flower whoe anther have een nipped off. 3 Analyi of phyical characteritic (ee Tale 10.1) of the offpring over 2 generation how evidence of hereditary tranmiion from oth parent. The tigma, where the pollen land, i at the tip of the carpel. Anther at the tip of the tamen are the ite of pollen production. tamen are the male ex organ. The ovary i the female ex organ. Pea pod Parent plant eed (pea) Mendel rought new method to experiment on inheritance Gregor Mendel wa an Autrian monk, not an academic cientit, ut he wa qualified to undertake cientific invetigation. Although in 1850 he had failed an examination for a teaching certificate in natural cience, he later undertook in A Controlled Cro etween Two Plant Plant were widely ued in early genetic tudie ecaue it i eay to control which individual mate with which. Mendel ued the garden pea (Pium ativum) in many of hi experiment.

3 GENETIC: MENDEL AND BEYOND 189 tenive tudie in phyic, chemitry, mathematic, and variou apect of iology at the Univerity of Vienna. Hi work in phyic and mathematic proaly led him to apply experimental and quantitative method to the tudy of heredity, and thee method were the key ingredient in hi ucce. Mendel worked out the aic principle of inheritance in plant over a period of aout 9 year. Hi work culminated in a pulic lecture in 1865 and a detailed written pulication in Mendel paper appeared in a journal that wa received y 120 lirarie, and he ent reprinted copie (of which he had otained 40) to everal ditinguihed cholar. However, hi theory wa not accepted. In fact, it wa ignored. The chief difficulty wa that the mot prominent iologit of Mendel time were not in the hait of thinking in mathematical term, even the imple term ued y Mendel. Even Charle Darwin, whoe theory of evolution y natural election depended on genetic variation among individual, failed to undertand the ignificance of Mendel finding. In fact, Darwin performed reeding experiment on napdragon imilar to Mendel on pea and got data imilar to Mendel, ut he mied the point, till relying on the concept of lending. In addition, Mendel had little crediility a a iologit; indeed, hi lowet grade were in iology! Whatever the reaon, Mendel pioneering paper had no dicernile influence on the cientific world for more than 30 year. Then, in 1900, after meioi had een oerved and decried, Mendel dicoverie urt into prominence a a reult of independent experiment y three plant geneticit, Hugo DeVrie, Carl Corren, and Erich von Tchermak. Each carried out croing experiment and otained quantitative data aout the progeny; each pulihed hi principal finding in 1900; each cited Mendel 1866 paper. They immediately realized that chromoome and meioi provided a phyical explanation for the theory that Mendel had propoed to explain the data from hi croe. A we go through Mendel work, we will decrie firt hi experiment and concluion, and then the chromoomal explanation of hi theorie. Mendel Experiment and the Law of Inheritance That Mendel wa ale to make hi dicoverie efore the dicovery of meioi wa due in part to the method of experimentation he ued. Mendel work i a fine example of preparation, choice of experimental material, execution, and interpretation. Let ee how he approached each of thee tep. Mendel devied a careful reearch plan Mendel choe the garden pea for hi tudie ecaue of it eae of cultivation, the feaiility of controlled pollination (ee Figure 10.1), and the availaility of varietie with differing trait. He controlled pollination, and thu fertilization, of hi parent plant y manually moving pollen from one plant to another. Thu he knew the parentage of the offpring in hi experiment. The pea plant Mendel tudied produce male and female ex organ and gamete in the ame flower. If untouched, they naturally elf-pollinate that i, the female organ of each flower receive pollen from the male organ of the ame flower. Mendel made ue of thi natural phenomenon in ome of hi experiment. Mendel egan y examining different varietie of pea in a earch for heritale character and trait uitale for tudy: A character i an oervale feature, uch a flower color. A trait i a particular form of a character, uch a white flower. A heritale character trait i one that i paed from parent to offpring. Mendel looked for character that had well-defined, contrating alternative trait, uch a purple flower veru white flower. Furthermore, thee trait had to e true-reeding, meaning that the oerved trait wa the only form preent for many generation. In other word, pea with white flower, when croed with one another, would have to give rie only to progeny with white flower for many generation; tall plant red to tall plant would have to produce only tall progeny. Mendel iolated each of hi true-reeding train y repeated inreeding (done y croing of iling plant that were eemingly identical or y allowing individual to elfpollinate) and election. In mot of hi work, Mendel concentrated on the even pair of contrating trait hown in Tale Before performing any experimental cro, he made ure that each potential parent wa from a true-reeding train an eential point in hi analyi of hi experimental reult. Mendel then collected pollen from one parental train and placed it onto the tigma (female organ) of flower of the other train whoe anther were removed. The plant providing and receiving the pollen were the parental generation, deignated P. In due coure, eed formed and were planted. The eed and the reulting new plant contituted the firt filial generation, or F 1. Mendel and hi aitant examined each F 1 plant to ee which trait it ore and then recorded the numer of F 1 plant expreing each trait. In ome experiment the F 1 plant were allowed to elf-pollinate and produce a econd filial generation, F 2. Again, each F 2 plant wa characterized and counted. In ummary, Mendel devied a well-organized plan of reearch, purued it faithfully and carefully, recorded great amount of quantitative data, and analyzed the numer he recorded to explain the relative proportion of the different kind of progeny. Hi reult and the concluion to which they led are the uject of the next everal ection.

4 190 CHAPTER TEN 10.1 Mendel Reult from Monohyrid Croe PARENTAL GENERATION PHENOTYPE F 2 GENERATION PHENOTYPE DOMINANT RECEIVE DOMINANT RECEIVE TOTAL RATIO pherical eed Wrinkled eed 5,474 1,850 7, :1 Yellow eed Green eed 6,022 2,001 8, :1 Purple flower White flower :1 Inflated pod Contricted pod , :1 Green pod Yellow pod :1 Axial flower Terminal flower :1 Tall tem Dwarf tem , :1 (1 m) (0.3 m) Mendel experiment 1 examined a monohyrid cro Experiment 1 in Mendel paper involved a monohyrid cro one involving offpring of a cro in which each memer of the P generation i true-reeding for a different trait. He took pollen from pea plant of a true-reeding train with wrinkled eed and placed it on the tigma of flower of a true-reeding train with pherical eed (Figure 10.2). He alo performed the reciprocal cro y placing pollen from the pherical-eeded train on the tigma of flower of the wrinkled-eeded train Contrating Trait In experiment 1, Mendel tudied the inheritance of eed hape. We know today that wrinkled eed poe an anormal form of tarch. In oth cae, all the F 1 eed produced were pherical it wa a if the wrinkled eed trait had diappeared completely. The following pring, Mendel grew 253 F 1 plant from thee pherical eed. Each of thee plant wa allowed to elf-pollinate to produce F 2 eed. In all, there were 7,324 F 2 eed, of which 5,474 were pherical and 1,850 wrinkled (Figure 10.3 and Tale 10.1). Mendel oerved that the wrinkled eed trait wa never expreed in the F 1 generation, even though it reappeared in the F 2 generation. He concluded that the pherical eed trait wa dominant to the wrinkled eed trait, which he called receive. In each of the other ix pair of trait Mendel tudied, one proved to e dominant over the other. Of mot importance, the ratio of the two trait in the F 2 generation wa alway the ame approximately 3:1. That i, three-fourth of the F 2 generation howed the dominant trait and one-fourth howed the receive trait (ee Tale 10.1). In Mendel experiment 1, the ratio wa 5,474:1,850 = 2.96:1. The reciprocal croe in the parental generation oth gave imilar outcome in the F 2 ; it did not matter which parent contriuted the pollen. By themelve, the reult from experiment 1 diproved the widely held elief that inheritance i alway a lending

5 EXPERIMENT Quetion: When two train with contrating trait reed, are their characteritic irreverily lended in ucceeding generation? METHOD Parental (P) eed 1 Parental (P) plant 2 3 REULT 4 P plant are cro-pollinated. F 1 eed F 1 eed are all pherical. F 1 plant elf-pollinate F 1 plant. Plant a true-reeding pherical eed Pollen F 2 eed from F 1 plant F 2 eed: 3 / 4 are pherical, 1/ 4 are wrinkled (3:1 ratio). Growth Pollen Plant a true-reeding wrinkled eed Maturation Growth Growth 2 Plant a pherical F 1 eed. Concluion: There i no irreverile lending of characteritic. A receive trait can reappear in ucceeding generation Mendel Experiment 1 The pattern Mendel oerved in the F 2 generation 1 4 of the eed wrinkled, 3 4 pherical wa the ame no matter which train contriuted the pollen in the parental generation. GENETIC: MENDEL AND BEYOND 191 phenomenon. According to the lending theory, Mendel F 1 eed hould have had an appearance intermediate etween thoe of the two parent in other word, they hould have een lightly wrinkled. Furthermore, the lending theory offered no explanation for the reappearance of the wrinkled trait in the F 2 eed after it apparent aence in the F 1 eed. Mendel propoed that the unit reponile for the inheritance of pecific trait are preent a dicrete particle that occur in pair and egregate (eparate) from one another during the formation of gamete. According to thi theory, the unit of inheritance retain their integrity in the preence of other unit. Thi particulate theory i in harp contrat to the lending theory, in which the unit of inheritance were elieved to loe their identitie when mixed together. A he worked mathematically with hi data, Mendel reached the tentative concluion that each pea plant ha two unit of inheritance for each character, one from each parent. During the production of gamete, only one of thee paired unit i given to a gamete. Hence each gamete contain one unit, and the reulting zygote contain two, ecaue it i produced y the fuion of two gamete. Thi concluion i the core of Mendel model of inheritance. Mendel unit of inheritance i now called a gene. Mendel reaoned that in experiment 1, the two truereeding parent plant had different form of the gene affecting eed hape. The pherical-eeded parent had two gene of the ame form, which we will call, and the parent with wrinkled eed had two gene. The parent produced gamete that each contained a ingle gene, and the parent produced gamete each with a ingle gene. Each memer of the F 1 generation had an from one parent and an from the other; an F 1 could thu e decried a. We ay that i dominant over ecaue the trait pecified y the allele i not evident when oth form of the gene are preent. The different form of a gene ( and in thi cae) are called allele. Individual that are true-reeding for a trait contain two copie of the ame allele. For example, all the individual in a population of a train of true-reeding pea with wrinkled eed mut have the allele pair ; if were preent, the plant would produce pherical eed. We ay that the individual that produce wrinkled eed are homozygou for the allele, meaning that they have two copie of the ame allele (). ome pea with pherical eed the one with the genotype are alo homozygou. However, not all plant with pherical eed have the genotype. ome pherical-eeded plant, like Mendel F 1, are heterozygou: They have two different allele of the gene in quetion (in thi cae, ). To illutrate thee term with a more complex example, one in which there are three gene pair, an individual with the genotype AABcc i homozygou for the A and C gene, ecaue it ha two A allele and two c allele, ut heterozygou for the B gene, ecaue it contain the B and allele. An individual that i homozygou for a character i ometime called a homozygote; a heterozygote i heterozygou for the character in quetion.

6 192 CHAPTER TEN The phyical appearance of an organim i it phenotype. Mendel correctly uppoed the phenotype to e the reult of the genotype, or genetic contitution, of the organim howing the phenotype. In experiment 1 we are dealing with two phenotype (pherical eed and wrinkled eed). The F 2 generation contain thee two phenotype, ut they are produced y three genotype. The wrinkled eed phenotype i produced only y the genotype, wherea the pherical eed phenotype may e produced y the genotype or. Parental (P) generation 1 A parent homozygou for the allele for pherical eed i croed with a parent homozygou for the allele for wrinkled eed. Mendel firt law ay that allele egregate How doe Mendel model of inheritance explain the compoition of the F 2 generation in experiment 1? Conider firt the F 1, which ha the pherical eed phenotype and the genotype. According to Mendel model, when any individual produce gamete, the two allele eparate, o that each gamete receive only one memer of the pair of allele. Thi i Mendel firt law, the law of egregation. In experiment 1, half the gamete produced y the F 1 generation contained the allele and half the allele. In the F 2 generation, ince oth and plant produce pherical eed while produce wrinkled eed, there are three way to get a pherical-eeded plant, ut only one way to get a wrinkled-eeded plant ( from oth parent) predicting a 3:1 ratio remarkaly cloe to the value Mendel found experimentally for all ix of the trait he compared (ee Tale 10.1). While thi imple example i eay to work out in your head, determination of expected allelic comination for more complicated inheritance pattern can e aided y ue of a Punnett quare, devied in 1905 y the Britih geneticit Reginald Crundall Punnett. Thi device remind u to conider all poile comination of gamete when calculating expected genotype frequencie. A Punnett quare look like thi: Female gamete Male gamete It i a imple grid with all poile male gamete genotype hown along one ide and all poile female gamete genotype along another ide. To complete the grid, we fill in each quare with the correponding pollen genotype and egg genotype, giving the diploid genotype of a memer of the F 2 generation. For example, to fill the rightmot quare, we put in the from the egg (female gamete) and the from the pollen (male gamete), yielding (Figure 10.4). Mendel did not live to ee hi theory placed on a ound phyical footing aed on chromoome and DNA. Gene are now known to e region of the DNA molecule in chromoome. More pecifically, a gene i a portion of the DNA that reide at a particular ite on a chromoome, called a locu F 1 generation F 2 generation Gamete Egg Gamete (plural, loci), and encode a particular character. Gene are expreed in the phenotype motly a protein with particular function, uch a enzyme. o a dominant gene can e thought of a a region of DNA that i expreed a a functional enzyme, while a receive gene typically expree a nonfunctional enzyme. Mendel arrived at hi law of egregation with no knowledge of chromoome or meioi, ut today we can picture the different allele of a gene egregating a chromoome eparate in meioi I (Figure 10.5). Mendel verified hi hypothei y performing a tet cro Mendel et out to tet hi hypothei that there were two poile allelic comination ( and ) in the pherical-eeded F 1 generation. He did o y performing a tet cro, which i a way of finding out whether an individual howing a dom- perm 2 The parental gamete comine to produce F 1 plant with the genotype and a pherical eed phenotype. 3 The heterozygou F 1 plant i elf-pollinated. 4 and gamete comine randomly to produce two different eed phenotype in the F 2 plant, a thi Punnett quare how Mendel Explanation of Experiment 1 Mendel concluded that inheritance depend on factor from each parent, and that thee factor are dicrete unit that do not lend in the offpring.

7 GENETIC: MENDEL AND BEYOND Thi ite on the chromoome i the locu of the gene with the allele and. 2 Before meioi I, each of the homologou chromoome replicate. Allele of gene for eed hape Homologou chromoome Diploid Parent Meiotic interphae For the eed hape gene that we have een conidering, the receive homozygote ued for the tet cro i. The individual eing teted may e decried initially a ecaue we do not yet know the identity of the econd allele. We can predict two poile reult: If the individual eing teted i homozygou dominant (), all offpring of the tet cro will e and how the dominant trait (pherical eed). If the individual eing teted i heterozygou (), then approximately half of the offpring of the tet cro will e heterozygou and how the dominant trait (), ut the other half will e homozygou for, and will how, the receive trait () (Figure 10.6). 3 At the end of meioi I, the two allele are egregated into eparate daughter cell. Meioi I EXPERIMENT Quetion: If an organim how a dominant phenotype, i it homozygou or heterozygou? pherical pea are of undetermined genotype. METHOD Wrinkled pea have a known genotype (homozygou receive). If the plant eing teted i homozygou If the plant eing teted i heterozygou Meioi II Gamete 4 Four haploid gamete At the end of meioi II, each haploid gamete contain one memer of each pair of homologou chromoome, and thu one allele for each pair of gene. Egg REULT perm Egg perm 10.5 Meioi Account for the egregation of Allele Although Mendel had no knowledge of chromoome or meioi, we now know that a pair of allele reide on homologou chromoome, and that meioi egregate thoe allele. then all progeny would how the dominant phenotype (pherical). then half the eed from the cro would e wrinkled, and half pherical. inant trait i homozygou or heterozygou. In a tet cro, the individual in quetion i croed with an individual known to e homozygou for the receive trait an eay individual to identify, ecaue in order to have the receive phenotype, it mut e homozygou for the receive trait. Concluion: The plant eing teted i homozygou. Concluion: The plant eing teted i heterozygou Homozygou or Heterozygou? An individual with a dominant phenotype may e homozygou or heterozygou. It genotype can e determined y croing it with a homozygou receive plant and oerving the phenotype of the progeny produced. Thi procedure i known a a tet cro.

8 194 CHAPTER TEN The econd prediction cloely matche the reult that Mendel otained; thu Mendel hypothei accurately predicted the reult of hi tet cro. With hi firt hypothei confirmed, Mendel went on to ak another quetion: How do different pair of gene ehave in croe when conidered together? Mendel econd law ay that allele of different gene aort independently Conider an organim that i heterozygou for two gene (Yy), in which the and Y allele came from it mother and and y came from it father. When thi organim produce gamete, do the allele of maternal origin ( and Y) go together to one gamete and thoe of paternal origin ( and y) to another gamete? Or can a ingle gamete receive one maternal and one paternal allele, and y (or and Y)? To anwer thee quetion, Mendel performed another erie of experiment. He egan with pea that differed in two eed character: eed hape and eed color. One true-reeding parental train produced only pherical, yellow eed (YY), and the other produced only wrinkled, green one (yy). A cro etween thee two train produced an F 1 generation in which all the plant were Yy. Becaue the and Y allele are dominant, the F 1 eed were all pherical and yellow. Mendel continued thi experiment to the F 2 generation y performing a dihyrid cro, which i a cro made etween individual that are identical doule heterozygote. There are two poile way in which uch douly heterozygou plant might produce gamete, a Mendel aw it. (Rememer that he had never heard of chromoome or meioi.) Firt, if the allele maintain the aociation they had in the parental generation (that i, if they are linked), then the F 1 plant hould produce two type of gamete (Y and y), and the F 2 progeny reulting from elf-pollination of the F 1 plant hould conit of three time a many plant earing pherical, yellow eed a one with wrinkled, green eed. Were uch reult to e otained, there might e no reaon to uppoe that eed hape and eed color were regulated y two different gene, ecaue pherical eed would alway e yellow and wrinkled one alway green. The econd poiility i that the egregation of from i independent of the egregation of Y from y (that i, that the two gene are not linked). In thi cae, four kind of gamete hould e produced y the F 1 in equal numer: Y, y, Y, and y. When thee gamete comine at random, they hould produce an F 2 of nine different genotype. The F 2 progeny could have any of three poile genotype for hape (,, or ) and any of three poile genotype for color (YY, Yy, or yy). The comined nine genotype hould produce jut four phenotype (pherical yellow, pherical green, wrinkled yellow, wrinkled green). By uing a Punnett quare, we can how that thee four phenotype would e expected to occur in a ratio of 9:3:3:1 (Figure 10.7). The reult of Mendel dihyrid croe matched the econd prediction: Four different phenotype appeared in the F 2 in a ratio of aout 9:3:3:1. The parental trait appeared in new comination (pherical green and wrinkled yellow). uch new comination are called recominant phenotype. Thee reult led Mendel to the formulation of what i now known a Mendel econd law: Allele of different gene aort independently of one another during gamete formation. That i, the egregation of the allele of gene A i independent of the egregation of the allele of gene B. We now know that thi law of independent aortment i not a univeral a the law of egregation, ecaue it applie to gene located on eparate chromoome, ut not necearily to thoe located YY Parental (P) generation F 1 generation F 2 generation Y When F 1 plant elf-pollinate, the gamete comine randomly to produce an F 2 generation with four phenotype in a 9:3:3:1 ratio. y Egg Y Yy y YY yy y Y Yy Yy Yy Yy Gamete YY yy YY yy Y Y yy Yy Yy Yy y YY yy y perm Y Yy 10.7 Independent Aortment The 16 poile comination of gamete in thi dihyrid cro reult in 9 different genotype. Becaue and Y are dominant over and y, repectively, the 9 genotype reult in 4 phenotype in a ratio of 9:3:3:1. Thee reult how that the two gene egregate independently. y

9 GENETIC: MENDEL AND BEYOND 195 Diploid parent Yy 1 When homolog line up on either ide of the metaphae plate during meioi 2 where and go Y Y Y y Four haploid gamete Y, y, y, Y on the ame chromoome, a we will ee elow. However, it i correct to ay that chromoome egregate independently during the formation of gamete, and o do any two gene on eparate homologou chromoome pair (Figure 10.8). One of Mendel major contriution to the cience of genetic wa hi ue of the rule of tatitic and proaility to analyze hi mae of data from hundred of croe producing thouand of plant. Hi mathematical analye led to clear pattern in the data, and then to hi hypothee. Ever ince Mendel, geneticit have ued imple mathematic in the ame way that Mendel did. Punnett quare or proaility calculation: A choice of method Punnett quare provide one way of olving prolem in genetic, and proaility calculation provide another. Many y y 3 doe not determine where Y and y go. 4 aociate with 5 aociate with Y and with y. Meioi continue in one of two orientation y and with Y. Y y 10.8 Meioi Account for Independent Aortment of Allele We now know that allele of different gene are egregated independently during metaphae I of meioi. Thu a parent of genotype Yy can form gamete with four different genotype. Y y y Y y Y people find it eaiet to ue the principle of proaility, perhap ecaue they are o familiar. When we flip a coin, the law of proaility tate that it ha an equal proaility of landing head or tail. For any given to of a fair coin, the proaility of head i independent of what happened in all the previou toe. A run of ten traight head implie nothing aout the next to. No law of average increae the likelihood that the next to will come up tail, and no momentum make an eleventh occurrence of head any more likely. On the eleventh to, the odd of getting head are till 50/50. The aic convention of proaility are imple: If an event i aolutely certain to happen, it proaility i 1. If it cannot poily happen, it proaility i 0. Otherwie, it proaility lie etween 0 and 1. A coin to reult in head approximately half the time, o the proaility of head i 1 2 a i the proaility of tail. MULTIPLYING PROBABILITIE. How can we determine the proaility of two independent event happening together? If two coin (a penny and a dime, ay) are toed, each act independently of the other. What, then, i the proaility of oth coin coming up head? Half the time, the penny come up head; of that fraction, half the time the dime alo come up head. Therefore, the joint proaility of oth coin coming up head i half of one-half, or = 1 4. To find the joint proaility of independent event, then, we multiply the proailitie of the individual event (Figure 10.9). How doe thi method apply to genetic? THE MONOHYBRID CRO. To apply the principle of proaility to genetic prolem, we need only deal with gamete formation and random fertilization intead of coin toe. A homozygote can produce only one type of gamete, o, for example, the proaility of an individual producing gamete with the genotype i 1. The heterozygote produce gamete with a proaility of 1 2, and gamete with a proaility of 1 2. Conider the F 2 progeny of the cro in Figure They are otained y elf-pollination of F 1 plant of genotype. The proaility that an F 2 plant will have the genotype mut e = 1 4, ecaue there i a 50:50 chance that the perm will have the genotype, and that chance i independent of the 50:50 chance that the egg will have the genotype. imilarly, the proaility of offpring i = 1 4. ADDING PROBABILITIE. How are proailitie calculated when an event can happen in different way? The proaility of an F 2 plant getting an allele from the perm and an allele from the egg i 1 4, ut rememer that the ame

10 196 CHAPTER TEN 2 1 Two coin toe are independent event with an outcome proaility (P) of 1 / 2 each. Thi outcome i the reult of thee two independent event. The joint proaility (P) = 1 / 2 i 1 / 2 1 / 2 = 1 / 4 (multiplication rule). P = 1/ 2 P = 1 / 2 1/ 2 1 / 2 = 1 / 4 P = 1/ 2 1/ 2 1 / 2 1/ 2 1 / 2 = 1 / 4 = 1 / 4 1/ 2 1 / 2 = 1 / 4 There are two way to arrive at a heterozygote, o we add the proailitie of the two individual outcome: 1/ / 4 = 1 / 2 (addition rule) Uing Proaility Calculation in Genetic The proaility of any given comination of allele from a perm and an egg appearing in the offpring of a cro can e otained y multiplying the proailitie of each event. ince a heterozygote can e formed in two way, thee two proailitie are added together. genotype can alo reult from an from the perm and an from the egg, alo with a proaility of 1 4. The proaility of an event that can occur in two or more different way i the um of the individual proailitie of thoe way. Thu the proaility that an F 2 plant will e a heterozygote i equal to the um of the proailitie of the two way of forming a heterozygote: = 1 2 (ee Figure 10.9). The three genotype are therefore expected in the ratio 1 4 : 1 2 : 1 4 hence the 1:2:1 ratio of genotype and the 3:1 ratio of phenotype een in Figure THE DIHYBRID CRO. If F 1 plant heterozygou for two independent character elf-pollinate, the reulting F 2 plant expre four different phenotype. The proportion of thee phenotype are eaily determined y proaility calculation. Let ee how thi work for the experiment hown in Figure Uing the principle decried aove, we can calculate that the proaility that an F 2 eed will e pherical i 3 4: the proaility of an heterozygote ( 1 2) plu the proaility of an homozygote ( 1 4) = 3 4. By the ame reaoning, the proaility that a eed will e yellow i alo 3 4. The two character are determined y eparate gene and are independent of each other, o the joint proaility that a eed will e oth pherical and yellow i = What i the proaility of F 2 eed eing oth wrinkled and yellow? The proaility of eing yellow i again 3 4; the proaility of eing wrinkled i = 1 4. The joint proaility that a eed will e oth wrinkled and yellow, then, i = The ame proaility applie, for imilar reaon, to pherical, green F 2 eed. Finally, the proaility that F 2 eed will e oth wrinkled and green i = Looking at all four phenotype, we ee they are expected in the ratio of 9:3:3:1. Proaility calculation and Punnett quare give the ame reult. Learn to do genetic prolem oth way, and then decide which method you prefer. Mendel law can e oerved in human pedigree After Mendel work wa uncovered y plant reeder, Mendelian inheritance wa oerved in human. Currently, the pattern of over 2,500 inherited human characteritic have een decried. How can Mendel law of inheritance e applied to human? Mendel worked out hi law y performing many planned croe and counting many offpring. Neither of thee approache i poile with human. o human geneticit rely on pedigree, family tree that how the occurrence of phenotype (and allele) in everal generation of related individual. Becaue human have uch mall numer of offpring, human pedigree do not how the clear proportion of offpring phenotype that Mendel aw in hi pea plant (ee Tale 10.1). For example, when two people who are oth heterozygou for a receive allele (ay, Aa) marry, there will e, for each of their children, a 25 percent proaility that the child will e a receive homozygote (aa). Thu, over many uch marriage, one-fourth of all the children will e receive homozygote (aa). But the offpring of a ingle marriage are likely to e too few to how the exact one-fourth proportion. In a family with only two children, for example, oth could eaily e aa (or Aa, or AA). To deal with thi amiguity, human geneticit aume that any allele that caue an anormal phenotype i rare in the human population. Thi mean that if ome memer of a given family have a rare allele, it i highly unlikely that an outider marrying into that family will have that ame rare allele. Human geneticit may wih to know whether a particular rare allele i dominant or receive. Figure depict a pedigree howing the pattern of inheritance of a rare domi-

11 GENETIC: MENDEL AND BEYOND 197 Generation I (parent) Generation II Generation III Every affected individual ha an affected parent. Aout 1 / 2 of the offpring (of oth exe) are affected. Female Male Mating Wild type Phenotype of interet Oldet Younget nant allele. The following are the key feature to look for in uch a pedigree: iling Pedigree Analyi and Dominant Inheritance Thi pedigree repreent a family affected y Huntington dieae, which reult from a rare dominant allele. Everyone who inherit thi allele i affected. Every affected peron ha an affected parent. Aout half of the offpring of an affected parent are alo affected. The phenotype occur equally in oth exe. Female Male Generation I (Parent) Generation II Generation III Generation IV Heterozygou for phenotype of interet (inferred) Mating etween relative 1 One parent i heterozygou 2 and the receive allele i paed on to one-half of the phenotypically normal offpring. 3 Thee couin are heterozygou. Compare thi pattern with Figure 10.11, which how the pattern of inheritance of a rare receive allele: Affected people uually have two parent who are not affected. In affected familie, aout one-fourth of the children of unaffected parent can e affected. The phenotype occur equally in oth exe. In pedigree howing inheritance of a receive phenotype, it i not uncommon to find a marriage of two relative. Thi pattern i a reult of the rarity of receive allele that give rie to anormal phenotype. For two phenotypically normal parent to have an affected child (aa), the parent mut oth e heterozygou (Aa). If a particular receive allele i rare in the general population, the chance of two people marrying who are oth carrying that allele i quite low. On the other hand, if that allele i preent in a family, two couin might hare it (ee Figure 10.10). Thi i why tudie on population iolated either culturally (y religion, a with the Amih in the United tate) or geographically (a on iland) have een o valuale to human geneticit. People in thee group tend to have large familie, or to marry among themelve, or oth. Becaue the major ue of pedigree analyi i in the clinical evaluation and couneling of patient with inherited anormalitie, a ingle pair of allele i uually followed. However, jut a pedigree analyi how the egregation of allele, it alo can how independent aortment if two different allele pair are conidered. 4 Mating of heterozygou receive parent may produce homozygou receive (affected) offpring Receive Inheritance Thi pedigree repreent a family that carrie the allele for alinim, a receive trait. Becaue the trait i receive, heterozygote do not have the alino phenotype, ut they can pa the allele on to their offpring. Affected peron mut inherit the allele from two heterozygou parent or (rarely) from one homozygou and one heterozygou parent. In thi family, the heterozygou parent are couin, ut the ame reult could occur if the parent were unrelated ut heterozygou. Allele and Their Interaction In many cae, allele do not how the imple relationhip etween dominance and receivene that we have decried. In other, a ingle allele may have multiple phenotypic effect. Exiting allele can give rie to new allele y mutation, o there can e many allele for a ingle character.

12 198 CHAPTER TEN New allele arie y mutation Different allele of a gene exit ecaue gene are uject to mutation, which are rare, tale, and inherited change in the genetic material. In other word, an allele can mutate to ecome a different allele. Mutation, which will e dicued in detail in Chapter 12, i a random proce; different copie of the ame allele may e changed in different way. One particular allele of a gene may e defined a the wild type, ecaue it i preent in mot individual in nature ( the wild ) and give rie to an expected trait or phenotype. Other allele of that gene, often called mutant allele, may produce a different phenotype. The wild-type and mutant allele reide at the ame locu and are inherited according to the rule et forth y Mendel. A genetic locu with a wild-type allele that i preent le than 99 percent of the time (the ret of the allele eing mutant) i aid to e polymorphic (from the Greek poly, many, and morph, form ). Many gene have multiple allele Becaue of random mutation, a group of individual may have more than two allele of a given gene. (Any one individual ha only two allele, of coure one from it mother and one from it father.) In fact, there are many example of uch multiple allele. Coat color in rait i determined y one gene with four allele. There i a dominance hierarchy among thee allele: C > c ch > c h > c Any rait with the C allele (paired with any of the four) i dark gray, and a rait with cc i alino. The intermediate color reult from the different allelic comination hown in Figure Multiple allele increae the numer of poile phenotype. In Mendel monohyrid cro, there wa jut one pair of allele () and two poile phenotype (reulting from or and ). The four allele of the rait coat color gene produce five phenotype. Dominance i not alway complete In the ingle pair of allele tudied y Mendel, dominance i complete when an individual i heterozygou. That i, an individual expree the phenotype. However, many gene have allele that are not dominant or receive to one another. Intead, the heterozygote how an intermediate phenotype at firt glance, like that predicted y the old lending theory of inheritance. For example, if a true-reeding red napdragon i croed with a true-reeding white one, all the F 1 flower are pink. That thi phenomenon can till e explained in term of Mendelian genetic, rather than lending, i readily demontrated y a further cro. The lending theory predict that if one of the pink F 1 napdragon i croed with a true-reeding white one, all the offpring hould e a till lighter pink. In fact, approximately 1 2 of the offpring are white, and 1 2 are the ame hade of pink a the F 1 parent. When the F 1 pink napdragon are allowed to elf-pollinate, the reulting F 2 plant are ditriuted in a ratio of 1 red:2 pink:1 white (Figure 10.13). Clearly the hereditary particle the gene have not lended; they are readily orted out in the F 2. We can undertand thee reult in term of the Mendelian law of inheritance. All we need to do i recognize that the heterozygote how a phenotype intermediate etween thoe of the two homozygote. In uch cae, the gene i aid to e governed y incomplete dominance. Incomplete dominance i common in nature. In fact, Mendel paper wa unuual in that all even of the example he decried (ee Tale 10.1) are characterized y complete dominance Inheritance of Coat Color in Rait There are four allele of the gene for coat color in rait. Different comination of two allele give different coat color. Poile genotype CC, Cc ch, Cc h, Cc c ch c ch c ch c h, c ch c c h c h, c h c cc Phenotype Dark gray Chinchilla Light gray Himalayan Alino

13 GENETIC: MENDEL AND BEYOND Parental (P) generation When true-reeding red and white parent are croed, the F 1 generation are all pink. F 1 generation 2 Heterozygou napdragon produce pink flower an intermediate phenotype ecaue the allele for red flower i incompletely dominant over the allele for white one. F 2 generation 3 When F 1 plant elfpollinate, they produce white, pink, and red F 2 offpring in a ratio of 1:2:1. Rr rr rr White RR Red Rr Rr Rr Rr rr Pink Pink Rr RR Pink Rr 1/ 4 White 1/ 2 Pink 1/ 4 Red 1/ 2 Pink White rr 1/ 2 White A tet cro confirm that pink napdragon are heterozygou Incomplete Dominance Follow Mendel Law An intermediate phenotype can occur in heterozygote when neither allele i dominant. The heterozygou phenotype (here, pink flower) may give the appearance of a lended trait, ut the trait of the parental generation reappear in their original form in ucceeding generation, a predicted y Mendel law of inheritance. Blood type of cell A B AB O Genotype I A I A or I A i O I B I B or I B i O I A I B i O i O Antiodie made y ody Anti-B Anti-A Neither anti-a nor anti-b Both anti-a and anti-b 4 Reaction to added antiodie Anti-A Anti-B Red lood cell that do not react with antiody remain evenly dipered. Red lood cell that react with antiody clump together (peckled appearance) ABO Blood Reaction Are Important in Tranfuion Red lood cell of type A, B, AB, and O were mixed with erum containing anti-a or anti-b antiodie. A you look down the column, note that each of the type, when mixed eparately with anti-a and with anti-b, give a unique pair of reult; thi i the aic method y which lood i typed. People with type O lood are good lood donor ecaue O cell do not react with either anti-a or anti-b antiodie. People with type AB lood are good recipient, ince they make neither type of antiody. In codominance, oth allele are expreed ometime the two allele at a locu produce two different phenotype that oth appear in heterozygote. An example of thi phenomenon, called codominance, i een in the ABO lood group ytem in human. Early attempt at lood tranfuion frequently killed the patient. Around 1900, the Autrian cientit Karl Landteiner mixed lood cell and erum (lood from which cell have een removed) from different individual. He found that only certain comination of lood are compatile. In other comination, the red lood cell from one individual form clump in the preence of erum from the other individual. Thi dicovery led to our aility to adminiter compatile lood tranfuion that do not kill the recipient. Clump form in incompatile tranfuion ecaue pecific protein in the erum, called antiodie, react with foreign, or nonelf, cell. The antiodie react with protein on the urface of nonelf cell, called antigen. Blood compatiility i determined y a et of three allele (I A, I B, and i O ) at one locu, which determine the antigen on the urface of red lood cell. Different comination of thee allele in different people produce four different lood type, or phenotype: A, B, AB, and O (Figure 10.14). The AB phenotype found in individual of I A I B genotype i an example of codominance thee individual produce cell urface antigen of oth the A and B type. ome allele have multiple phenotypic effect Mendel principle were further extended when it wa dicovered that a ingle allele can reult in more than one phenotype. When a ingle allele ha more than one ditinguihale phenotypic effect, we ay that the allele i pleiotropic. A familiar example of pleiotropy involve the allele reponile for the coloration pattern (light ody, darker extremitie) of iamee cat, dicued later in thi chapter. The ame allele i alo reponile for the characteritic croed eye of iamee cat. Although thee effect appear to e unrelated, oth reult from the ame protein produced under the influence of the allele.

14 200 CHAPTER TEN Gene Interaction Thu far we have treated the phenotype of an organim, with repect to a given character, a a imple reult of the allele of a ingle gene. In many cae, however, everal gene interact to determine a phenotype. To complicate thing further, the phyical environment may interact with the genetic contitution of an individual in determining the phenotype. ome gene alter the effect of other gene Epitai occur when the phenotypic expreion of one gene i affected y another gene. For example, everal gene determine coat color in mice. The wild-type color i agouti, a grayih pattern reulting from and on the individual hair. The dominant allele B determine that the hair will have and and thu that the color will e agouti, wherea the homozygou receive genotype reult in unanded hair, and the moue appear lack. A econd locu, on another chromoome, affect an early tep in the formation of hair pigment. The dominant allele A at thi locu allow normal color development, ut aa lock all pigment production. Thu, aa mice are all-white alino, irrepective of their genotype at the B locu (Figure 10.15). If a moue with genotype AABB (and thu the agouti phenotype) i croed with an alino of genotype aa, the F 1 Mice with genotype aa are alino regardle of their genotype for the other locu, ecaue the aa genotype lock all pigment production. Mice with genotype are lack unle they are alo aa (which make them alino). Mice that have at leat one dominant allele at each locu are agouti Gene May Interact Epitatically Epitai occur when one gene alter the phenotypic effect of another gene. In thee mice, the preence of the receive genotype (aa) at one locu lock pigment production, producing an alino moue no matter what the genotype i at the econd locu. mice are AaB and have the agouti phenotype. If the F 1 mice are croed with each other to produce an F 2 generation, then epitai will reult in an expected phenotypic ratio of 9 agouti:3 lack:4 alino. (Can you how why? The underlying ratio i the uual 9:3:3:1 for a dihyrid cro with unlinked gene, ut look cloely at each genotype, and watch out for epitai.) In another form of epitai, two gene are mutually dependent: The expreion of each depend on the allele of the other. The epitatic action of uch complementary gene may e explained a follow: uppoe gene A code for enzyme A in the metaolic pathway for purple pigment in flower, and gene B code for enzyme B: colorle precuror enzyme A colorle intermediate enzyme B purple pigment In order for the pigment to e produced, oth reaction mut take place. The receive allele a and code for nonfunctional enzyme. If a plant i homozygou for either a or, the correponding reaction will not occur, no purple pigment will form, and the flower will e white. Hyrid vigor reult from new gene comination and interaction If Mendel paper wa the mot important event in genetic in the nineteenth century, perhap an equally important paper in applied genetic wa pulihed early in the twentieth century y G. H. hull, titled The compoition of a field of maize. Farmer growing crop have known for centurie that mating among cloe relative (known a inreeding) can reult in offpring of lower quality than thoe from mating etween unrelated individual. The reaon for thi i that cloe relative tend to have the ame receive allele, ome of which may e harmful, a we aw in our dicuion of human pedigree aove. In fact, it ha long een known that if one croe two true-reeding, homozygou genetic train of a plant or animal, the reult i offpring that are phenotypically much tronger, larger, and in general more vigorou than either of the parent (Figure 10.16). hull egan hi experiment with two of the thouand of exiting varietie of corn (maize). Both varietie produced aout 20 uhel of corn per acre. But when he croed them, the yield of their offpring wa an atonihing 80 uhel per acre. Thi phenomenon i known a heteroi (hort for heterozygoi), or hyrid vigor. The cultivation of hyrid corn pread rapidly in the United tate and all over the world, quadrupling grain production. The practice of hyridization ha pread to many other crop and animal ued in agriculture. The actual mechanim y which heteroi work i not known. A widely accepted hypothei i overdominance, in

15 GENETIC: MENDEL AND BEYOND 201 Parent Parent Hyrid offpring Hyrid Vigor in Corn The heterozygou F 1 offpring i larger and more vigorou than either homozygou parent. which the heterozygou condition in certain important gene i uperior to either homozygote. Mot complex phenotype are determined y multiple gene and environment The difference etween individual organim in imple character, uch a thoe that Mendel tudied in pea, are dicrete and qualitative. For example, the individual in a population of pea are either hort or tall. For mot complex character, however, uch a height in human, the phenotype varie more or le continuouly over a range. ome people are hort, other are tall, and many are in etween the two extreme. uch variation within a population i called quantitative, or continuou, variation. In mot cae, quantitative variation i due to two factor (Figure 10.17): multiple gene, each with multiple allele, and environmental influence on the expreion of thee gene. Geneticit call the gene that together determine a complex character quantitative trait loci. Identifying thee loci i a major challenge, and an important one. For example, the amount of grain that a variety of rice produce in a growing eaon i determined y many interacting genetic factor. Crop plant reeder have worked hard to decipher thee fac- The environment affect gene action The phenotype of an individual doe not reult from it genotype alone. Genotype and environment interact to determine the phenotype of an organim. Environmental variale uch a light, temperature, and nutrition can affect the tranlation of a genotype into a phenotype. A familiar example of thi phenomenon involve the iamee cat. Thi handome animal normally ha darker fur on it ear, noe, paw, and tail than on the ret of it ody. Thee darkened extremitie normally have a lower temperature than the ret of the ody. A few imple experiment how that the iamee cat ha a genotype that reult in dark fur, ut only at temperature elow the general ody temperature. If ome dark fur i removed from the tail and the cat i kept at higher than uual temperature, the new fur that grow in i light. Converely, removal of light fur from the ack, followed y local chilling of the area, caue the pot to fill in with dark fur. Two parameter decrie the effect of gene and environment on the phenotype: Penetrance i the proportion of individual in a group with a given genotype that actually how the expected phenotype. Expreivity i the degree to which a genotype i expreed in an individual. For an example of environmental effect on expreivity, conider how iamee cat kept indoor or outdoor in different climate might look. Numer of individual aa Aa AA Aa Variation in phenotype Quantitative inheritance: 3 phenotypic clae Quantitative phenotype due to environment Interaction of gene and environment produce continuou variation Quantitative Variation Quantitative variation i produced y the interaction of gene and environment. In thi illutration, only a ingle gene with three allele i conidered. Mot complex character are determined y many gene and allele, with the environment exerting an influence on each.

16 202 CHAPTER TEN tor in order to reed higher-yielding rice train. In a imilar way, human characteritic uch a dieae uceptiility and ehavior are caued in part y quantitative trait loci. Gene and Chromoome The recognition that gene occupy characteritic poition on chromoome and are egregated y meioi enaled Mendel ucceor to provide a phyical explanation for hi model of inheritance. It oon ecame apparent that the aociation of gene with chromoome ha other genetic conequence a well. We mentioned aove that gene located on the ame chromoome may not follow Mendel law of independent aortment. What i the pattern of inheritance of uch gene? How do we determine where gene are located on a chromoome, and the ditance etween them? The anwer to thee and many other genetic quetion were worked out in tudie of the fruit fly Droophila melanogater. It mall ize, it eae of cultivation, and it hort generation time made thi animal an attractive experimental uject. Beginning in 1909, Thoma Hunt Morgan and hi tudent pioneered the tudy of Droophila in Columia Univerity famou fly room, where they dicovered the phenomena decried in thi ection. Droophila remain extremely important in tudie of chromoome tructure, population genetic, the genetic of development, and the genetic of ehavior. Gene on the ame chromoome are linked ome of the croe Morgan performed with fruit flie reulted in phenotypic ratio that were not in accord with thoe predicted y Mendel law of independent aortment. Morgan croed Droophila of two known genotype, BVgvg vgvg, for two different character, ody color and wing hape: B (wild-type gray ody), i dominant over (lack ody) Vg (wild-type wing) i dominant over vg (vetigial, a very mall wing) (Do you recognize thi type of cro? It i a tet cro for the two gene pair; ee Figure 10.6.) Morgan expected to ee four phenotype in a ratio of 1:1:1:1, ut that i not what he oerved. The ody color gene and the wing ize gene were not aorting independently; rather, they were for the mot part inherited together (Figure 10.18). Thee reult ecame undertandale to Morgan when he aumed that the two loci are on the ame chromoome that i, that they are linked. After all, ince the numer of gene in a cell far exceed the numer of chromoome, each chromoome mut contain many gene. The full et of loci on a given chromoome contitute a linkage group. The numer of linkage group in a pecie equal the numer of homologou chromoome pair. EXPERIMENT Quetion: Do allele for different characteritic alway aort independently? Parent (P) BVgvg Wild type (gray ody, normal wing) å vgvg (Black ody, vetigial wing) ç F 1 Thee are the reult expected from Mendel econd law (independent aortment) Genotype Expected reult Oerved phenotype (numer of individual) BVgvg Wild type 575 vgvg Black vetigial Parental phenotype 575 Bvgvg Gray vetigial 575 Vgvg Black normal Recominant phenotype ut the actual reult were inconitent with the law. Concluion: Thee two gene do not aort independently. They are linked on the ame chromoome ome Allele Do Not Aort Independently Morgan tudie howed that the gene for ody color and wing ize in Droophila are linked, o their allele do not aort independently. Linkage account for the departure of the phenotype ratio oerved from the reult predicted y Mendel law of independent aortment.

17 GENETIC: MENDEL AND BEYOND 203 uppoe, now, that the B and Vgvg loci are indeed located on the ame chromoome. Why, then, didn t all of Morgan F 1 flie have the parental phenotype that i, why did hi cro reult in anything other than gray flie with normal wing (wild-type) and lack flie with vetigial wing? If we aumed that linkage i aolute that i, that chromoome alway remain intact and unchanged we would expect to ee jut thoe two type of progeny. However, thi i not alway what happen. Gene can e exchanged etween chromatid Aolute linkage i extremely rare. If linkage were aolute, Mendel law of independent aortment would apply only to loci on different chromoome. What actually happen i more complex, and therefore more intereting. Chromoome are not unreakale, o recomination of gene can occur. That i, gene at different loci on the ame chromoome do ometime eparate from one another during meioi. Gene may recomine when two homologou chromoome phyically exchange correponding egment during prophae I of meioi that i, y croing over (Figure 10.19; ee alo Figure 9.16). Recall from Chapter 9 that the DNA i replicated during the phae, o that y prophae I, when homologou chromoome pair come together to form tetrad, each chromoome conit of two chromatid. The exchange event involve only two of the four chromatid in a tetrad, one from each memer of the homologou pair, and can occur at any point along the length of the chromoome. The chromoome egment involved are exchanged reciprocally, o oth chromatid involved in croing over ecome recominant (that i, each chromatid end up with gene from oth of the organim parent). Uually everal exchange event occur along the length of each homologou pair. When croing over take place etween two linked gene, not all progeny of a cro will have the parental phenotype. Intead, recominant offpring appear a well, a they did in Morgan cro. They appear in proportion called recominant frequencie, which are calculated y dividing the numer of recominant progeny y the total numer of progeny (Figure 10.20). Recominant frequencie will e greater for loci that are farther apart on the chromoome than for loci that are cloer together, ecaue an exchange event i more likely to occur etween gene that are far apart than etween gene that are cloe together. Geneticit can make map of chromoome If two loci are very cloe together on a chromoome, the odd of croing over etween them are mall. In contrat, if two loci are far apart, croing over could occur etween them at Homologou chromoome Gene at different loci on the ame chromoome can eparate and recomine y croing over. Croover Meioi I continue Recominant chromoome vg vg vg vg Meioi II Vg Vg B Vg vg Meioi I Gamete many point. In a population of cell undergoing meioi, a greater proportion of the cell will undergo recomination etween two loci that are far apart than etween two loci that are cloe together. In 1911, Alfred turtevant, then an undergraduate tudent in T. H. Morgan fly room, realized how that imple inight could e ued to how where different gene lie on a chromoome in relation to one another. B vg Vg B vg B Vg B Vg B B Vg vg The reult i two recominant gamete from each event of croing over Croing Over Reult in Genetic Recomination Gene at different loci on the ame chromoome can e eparated from one another and recomined y croing over. uch recomination occur during prophae I of meioi. B Vg

18 204 CHAPTER TEN Gray normal (wild type) B å Vg vg Black vetigial ç vg vg Recominant Frequencie The frequency of recominant offpring (thoe with a phenotype different from either parent) can e calculated. Recominant frequencie will e larger for loci that are far apart than for thoe that are cloe together on the chromoome. Recomination Parental genotype Recominant genotype vg B Vg B vg Vg vg vg B Vg B vg Vg vg vg vg vg Black vetigial Wild type Gray vetigial Black normal Numer of individual Parental phenotype Recominant (nonparental) phenotype 391 recominant Recominant frequency = = ,300 total offpring y i choen a an aritrary reference point, 0. Yellow ody White eye The Morgan group had determined recominant frequencie for many pair of linked gene. turtevant ued thee recominant frequencie to create genetic map that howed the arrangement of gene along the chromoome (Figure 10.21). Ever ince turtevant demontrated thi method, geneticit have mapped the chromoome of eukaryote, prokaryote, and virue, aigning ditance etween gene in map unit. A map unit correpond to a recominant frequency of 0.01; it i alo referred to a a centimorgan (cm), in honor of the founder of the fly room. You, too, can work out a genetic map (Figure 10.22) tep toward a Genetic Map Becaue the chance of a recominant genotype occurring increae with the ditance etween two loci on a chromoome, turtevant wa ale to derive thi partial map of a Droophila chromoome from the Morgan group data on the recominant frequencie of five receive trait. He ued an aritrary unit of ditance the map unit, or centimorgan (cm) equivalent to a recominant frequency of Vermilion eye Miniature wing Rudimentary wing Chromoome Genetic map in map unit (cm) Recominant frequencie yw v m r y and w = w and v = y and v = w and m = y and m = v and m = v and r = 0.269

19 GENETIC: MENDEL AND BEYOND At the outet, we have no idea of the individual ditance, and there are everal poile equence (a--c, a-c-, -a-c) Map Thee Gene The oject of thi exercie i to determine the order of three loci (a,,and c) on a chromoome, a well a the map ditance (in cm) etween them. a c a a We make a cro AABB aa, and otain an F 1 generation with a genotype AaB. We tet cro thee AaB individual with aa. Here are the genotype of the firt 1,000 progeny: 450 AaB, 450 aa, 50 Aa, and 50 aab. c c 4 How far apart are the and c gene? We make a cro BBCC cc, otain an F 1 generation, and tet cro it, otaining: 490 BCc, 490 cc, 10 Bcc, and 10 Cc. Determine the map ditance etween and c. 2 How far apart are the a and gene? Well, what i the recominant frequency? Which are the recominant type, and which are the parental type? Recominant frequency ( to c) = ( )/1,000 = 0.02 Map ditance = 100 recominant frequency = = 2 cm Recominant frequency (a to ) = ( )/1,000 = 0.1 o the map ditance i Map ditance = 100 recominant frequency = = 10 cm a 10 cm 5 Which of the three gene i etween the other two? Becaue a and are the farthet apart, c mut e etween them. 10 cm 2 cm c 3 Now we make a cro AACC aacc, otain an F 1 generation, and tet cro it, otaining: a 8 cm c 2 cm 460 AaCc, 460 aacc, 40 Aacc, and 40 aacc. How far apart are the a and c gene? Thee numer add up perfectly, ut in mot real cae they don't add up perfectly ecaue of multiple croover. Recominant frequency (a to c) = ( )/1,000 = 0.08 Map ditance = 100 recominant frequency = = 8 cm a 8 cm ex Determination and ex-linked Inheritance In Mendel work, reciprocal croe alway gave identical reult; it did not matter, in general, whether a dominant allele wa contriuted y the mother or y the father. But in ome cae, the parental origin of a chromoome doe matter. For example, a we aw at the eginning of thi chapter, human male inherit hemophilia A from their mother, not from their father. To undertand the type of inheritance in which the parental origin of an allele i important, we mut conider the way in which ex i determined in different pecie. c ex i determined in different way in different pecie In corn, a plant much tudied y geneticit, every diploid adult ha oth male and female reproductive tructure. The tiue in thee two type of tructure are genetically identical, jut a root and leave are genetically identical. Plant uch a corn, in which the ame individual produce oth male and female gamete, are aid to e monoeciou (from the Greek, one houe ). Other plant, uch a date palm and oak tree, and mot animal are dioeciou ( two houe ), meaning that ome individual can produce only male gamete and the other can produce only female gamete. In other word, dioeciou organim have two exe. In mot dioeciou organim, ex i determined y difference in the chromoome, ut uch determination operate in different way in different group of organim. For example, the ex of a honeyee depend on whether it develop from a fertilized or an unfertilized egg. A fertilized egg i diploid and give rie to a female ee either a worker or a queen, depending on the diet during larval life (again, note how the environment affect the phenotype). An unfertilized egg i haploid and give rie to a male drone: å å ç Diploid worker Diploid queen Haploid drone

20 206 CHAPTER TEN In many other animal, including human, ex i determined y a ingle ex chromoome, or y a pair of them. Both male and female have two copie of each of the ret of the chromoome, which are called autoome. Female grahopper, for example, have two X chromoome, wherea male have only one. Female grahopper are decried a eing XX (ignoring the autoome) and male a XO (pronounced ex-oh ): XX å ç Female form egg that contain one copy of each autoome and one X chromoome. Male form approximately equal amount of two type of perm: One type contain one copy of each autoome and one X chromoome; the other type contain only autoome. When an X-earing perm fertilize an egg, the zygote i XX, and develop into a female. When a perm without an X fertilize an egg, the zygote i XO, and develop into a male. Thi chromoomal mechanim enure that the two exe are produced in approximately equal numer. A in grahopper, female mammal have two X chromoome and male have one. However, male mammal alo have a ex chromoome that i not found in female: the Y chromoome. Female may e repreented a XX and male a XY: XX å Male produce two kind of gamete. Each gamete ha a complete et of autoome, ut half the gamete carry an X chromoome and the other half carry a Y. When an X-earing perm fertilize an egg, the reulting XX zygote i female; when a Y-earing perm fertilize an egg, the reulting XY zygote i male. XY ç X The X and Y chromoome have different function ome utle ut important phenotypic difference how up clearly in mammal with anormal ex chromoome contitution. Thee condition, which reult from nondijunction, a decried in Chapter 9, tell u omething aout the function of the X and Y chromoome. In human, XO individual ometime appear. Human XO individual are female who are phyically moderately anormal ut mentally normal; uually they are alo terile. The XO condition in human i called Turner yndrome. It i the only known cae in which a human can urvive with only one memer of a chromoome pair (here, the XY pair), although mot XO conception terminate pontaneouly early in development. XXY individual alo occur; thi condition i known a Klinefelter yndrome. People with thi genotype are ometime taller than average, alway terile, and alway male. Thee oervation uggeted that the gene that determine malene i located on the Y chromoome. Oervation of people with other type of chromoomal anormalitie helped reearcher to pinpoint the location of that gene: ome XY individual are phenotypically women and lack a mall portion of the Y chromoome. ome men are genetically XX and have a mall piece of the Y chromoome preent ut attached to another chromoome. The Y fragment that i miing and preent in thee two example, repectively, contain the malene-determining gene, which wa named RY (ex-determining region on the Y chromoome). The RY gene encode a protein involved in primary ex determination that i, the determination of the kind of gamete that will e produced and the organ that will make them. In the preence of functional RY protein, the emryo develop perm-producing tete. (Notice that italic type i ued for the name of a gene, ut roman type i ued for the name of a protein.) If the emryo ha no Y chromoome, the RY gene i aent, and thu the RY protein i not made. In the aence of the RY protein, the emryo develop egg-producing ovarie. In thi cae, a gene on the X chromoome called DAX1 produce an anti-teti factor. o the role of RY in a male i to inhiit the malene inhiitor encoded y DAX1. The RY protein doe thi in male cell, ut ince it i not preent in female, DAX1 can act to inhiit malene. Primary ex determination i not the ame a econdary ex determination, which reult in the outward manifetation of malene and femalene (ody type, reat development, ody hair, and voice). Thee outward characteritic are not determined directly y the preence or aence of the Y chromoome. Rather, they are determined y gene cattered on the autoome and X chromoome that control the action of hormone, uch a tetoterone and etrogen. The Y chromoome function differently in Droophila melanogater. uperficially, Droophila follow the ame pattern of ex determination a mammal female are XX and male are XY. However, XO individual are male (rather than female a in mammal) and almot alway are inditinguihale from normal XY male except that they are terile. XXY Droophila are normal, fertile female: XX å Fertile X ç terile Fertile XXY å Fertile XY ç

21 GENETIC: MENDEL AND BEYOND 207 Thu, in Droophila, ex i determined y the ratio of X chromoome to autoome et. If there i one X chromoome for each et of autoome, the individual i a female; if there i only one X chromoome for the two et of autoome, the individual i a male. The Y chromoome play no ex-determining role in Droophila, ut it i needed for male fertility. Caenorhaditi elegan i a favorite model organim for tudie of development (ee Chapter 19). Thi tiny worm ha two exe: male and hermaphrodite (elf-fertilizing). A in fruit flie, ex i determined y the X:autoome ratioindividual with a ratio elow 0.67 are male. In ird, moth, and utterflie, male are XX and female are XY. To avoid confuion, thee form are uually expreed a ZZ (male) and ZW (female): ZW å In thee organim, the female produce two type of gamete, carrying Z or W. Whether the egg i Z or W determine the ex of the offpring, in contrat to human and fruit flie, in which the perm, carrying either X or Y, determine the ex. ZZ ç Gene on ex chromoome are inherited in pecial way Gene on ex chromoome do not how the Mendelian pattern of inheritance we have decried aove. In Droophila and in human, the Y chromoome carrie few known gene, ut a utantial numer of gene affecting a great variety of character are carried on the X chromoome. Any uch gene i preent in two copie in female, ut in only one copy in male. Therefore, female may e heterozygou for gene that are on the X chromoome, ut male will alway e hemizygou for gene on the X chromoome they will have only one copy of each, and it will e expreed. Thu, reciprocal croe do not give identical reult for character whoe gene are carried on the ex chromoome, and thee character do not how the uual Mendelian ratio for the inheritance of gene located on autoome. The firt and till one of the et example of inheritance of character governed y loci on the ex chromoome (exlinked inheritance) i that of eye color in Droophila. The wild-type eye color of thee flie i red. In 1910, Morgan dicovered a mutation that caue white eye. He experimented y croing flie of the wild-type and mutant phenotype. Hi reult demontrated that the eye color locu i on the X chromoome. tudy Figure a you follow the croe and reult: (a) Wild-type allele Allele for white eye No allele at all () Homozygou red-eyed female Hemizygou white-eyed male Homozygou white-eyed female X X X Y X X X Y Hemizygou red-eyed male å ç å ç Egg perm Egg perm Eye Color I a ex-linked Trait in Droophila Thoma Hunt Morgan demontrated that a mutant allele that caue white eye in Droophila i carried on the X chromoome. Note that in thi cae, the reciprocal croe do not have the ame reult. All daughter are red-eyed heterozygote. å ç All on are red-eyed hemizygote. All daughter are red-eyed heterozygote. å ç All on are white-eyed hemizygote.

22 208 CHAPTER TEN When a homozygou red-eyed female wa croed with a (hemizygou) white-eyed male, all the on and daughter had red eye, ecaue red i dominant over white and all the progeny had inherited a wild-type X chromoome from their mother (Figure 10.23a). However, in the reciprocal cro, in which a white-eyed female wa mated with a red-eyed male, all the on were white-eyed and all the daughter were red-eyed (Figure 10.23). The on from the reciprocal cro inherited their only X chromoome from their white-eyed mother; the Y chromoome they inherited from their father doe not carry the eye color locu (Figure 10.23). The daughter, on the other hand, got an X chromoome earing the white allele from their mother and an X chromoome earing the red allele from their father; they were therefore red-eyed heterozygote (Figure 10.23). When heterozygou female were mated with red-eyed male, half their on had white eye, ut all their daughter had red eye. Together, thee reult howed that eye color wa carried on the X chromoome and not on the Y. Human diplay many ex-linked character The human X chromoome carrie aout two thouand gene. The allele at thee loci follow the ame pattern of inheritance a thoe for white eye in Droophila. One human X chromoome gene, for example, ha a mutant receive allele that lead to red-green color lindne, a hereditary diorder. Red-green color lindne appear in individual who are homozygou or hemizygou for the mutant allele. Pedigree analyi of X-linked receive phenotype (Figure 10.24) reveal the following pattern: The phenotype appear much more often in male than in female, ecaue only one copy of the rare allele i needed for it expreion in male, while two copie mut e preent in female. A male with the mutation can pa it on only to hi daughter; all hi on get hi Y chromoome. Daughter who receive one mutant X chromoome are heterozygou carrier. They are phenotypically normal, ut they can pa the mutant X to oth on and daughter (ut do o only half of the time, on average, ince half of their X chromoome carry the normal allele). The mutant phenotype can kip a generation if the mutation pae from a male to hi daughter (who will e phenotypically normal) and thu to her on. Hemophilia A, which affected the family decried at the eginning of thi chapter, i an X-linked receive phenotype, a are everal other important human dieae, a we will ee in later chapter. Human mutation inherited a X-linked dominant phenotype are rarer than X-linked receive ecaue dominant phenotype appear in every generation, and ecaue people carrying the harmful mutation, even a heterozygote, often fail to urvive and reproduce. (Look at the four point aove and try to determine what would happen if the mutation were dominant.) The mall human Y chromoome carrie everal dozen gene. Among them i the malene determinant, RY. Interetingly, for ome gene on the Y, there are imilar, ut not identical, gene on the X. For example, one of the protein Red-Green Color Blindne i a ex-linked Trait in Human The mutant allele for red-green color lindne i inherited a an X-linked receive. Female who carrie gene for phenotype of interet on one X chromoome Thi woman carrie the mutant allele ut he i a phenotypically normal heterozygote. Thi woman inherited the mutant X from her mother and a normal X from her father. Generation I (Parent) Generation II Generation III Generation IV Thi man inherited the mutant X chromoome from hi mother and a normal Y from hi father, and expree the mutation. He paed hi mutant X chromoome to hi daughter, and he paed it on to her on. Two iling inherited the mutant X from their mother. The on expree the mutation; hi iter i a carrier. In thi tet for red-green color lindne, people with normal color viion will ee the numer 15.

23 GENETIC: MENDEL AND BEYOND 209 that make up rioome ha a gene on the Y that i expreed only in male cell, while the X-linked counterpart i expreed in oth exe. Thi mean that there are male and female rioome; the ignificance of thi phenomenon i unknown. Y-linked allele are paed only from father to on. (You can verify thi with a Punnett quare.) Non-Nuclear Inheritance The nucleu i not the only organelle in a eukaryotic cell that carrie genetic material. A we decried in Chapter 4, mitochondria and platid, which may have arien from prokaryote that colonized other cell, contain mall numer of gene. For example, in human, there are aout 30,000 gene in the nuclear genome and 37 in the mitochondrial genome. Platid genome are aout five time larger than thoe of mitochondria. In any cae, everal of the gene of cytoplamic organelle are important for organelle aemly and function, o it i not urpriing that mutation of thee gene have profound effect on the organim. The inheritance of organelle gene differ from that of nuclear gene for everal reaon: In mot organim, mitochondria and platid are inherited from the mother only. A you will ee in later chapter, egg contain aundant cytoplam and organelle, ut the only part of the perm that urvive to take part in the union of haploid gamete i the nucleu. o you have inherited your mother mitochondria (with their gene), ut not your father. There may e hundred of mitochondria or platid in a cell. o a cell i not diploid for organelle gene; rather, it i highly polyploid. Organelle gene tend to mutate at much fater rate than nuclear gene, o there are multiple allele of organelle gene. The phenotype of mutation in the DNA of organelle reflect the organelle role. For example, in plant and ome eukaryotic algae, certain platid mutation affect the protein that aemle chlorophyll molecule into photoytem (ee Figure 8.9) and reult in a phenotype that i eentially white intead of green. Mitochondrial mutation that affect one of the complexe in the electron tranport chain reult in le ATP production. They have epecially noticeale effect in tiue with a high energy requirement, uch a the nervou ytem, mucle, and kidney. In 1995, Greg Lemond, a profeional cyclit who had won the famou Tour de France three time, wa forced to retire ecaue of mucle weakne upected to e caued y a mitochondrial mutation. Chapter ummary The Foundation of Genetic Although it had long een known that oth parent contriute to the character trait of their offpring, efore Mendel time it wa elieved that, once they were rought together, the unit of inheritance lended and could never e eparated. Although Gregor Mendel work wa meticulou and well documented, hi dicoverie, reported in the 1860, were ignored until decade later. Mendel Experiment and the Law of Inheritance Mendel ued the garden pea for hi tudie ecaue the plant were eaily cultivated and croed and ecaue they howed numerou character (uch a eed hape) with clearly different trait (pherical or wrinkled). Review Figure 10.1, Tale 10.1 In a monohyrid cro, the offpring of the firt generation (F 1 ) howed only one of the two parental trait. Mendel propoed that the trait oerved in the F 1 wa dominant and the other wa receive. Review Tale 10.1 When the F 1 offpring were elf-pollinated, the reulting F 2 generation howed a 3:1 phenotypic ratio, with the receive phenotype preent in one-fourth of the offpring. Thi reappearance of the receive phenotype refuted the lending theory. Review Figure 10.3 Becaue ome allele are dominant and ome are receive, the ame phenotype can reult from different genotype. Homozygou genotype have two copie of the ame allele; heterozygou genotype have two different allele. Heterozygou genotype yield phenotype that how the dominant trait. On the ai of many croe uing different character, Mendel propoed hi firt law: that the unit of inheritance (now known a gene) are particulate, that there are two allele of each gene in each parent, and that during gamete formation the two allele egregate from each other. Review Figure 10.4 Geneticit who followed Mendel howed that gene are carried on chromoome and that allele are egregated during meioi I. Review Figure 10.5 Uing a tet cro, Mendel wa ale to determine whether a plant howing the dominant phenotype wa homozygou or heterozygou. The appearance of the receive phenotype in half of the offpring of uch a cro indicate that the parent i heterozygou. Review Figure ee We/CD Activity 10.1 From tudie of the inheritance of two character uing dihyrid croe, Mendel concluded that allele of different gene aort independently. Review Figure 10.7, ee We/CD Tutorial 10.1 We can predict the reult of hyrid croe either y uing a Punnett quare or y calculating proailitie. To determine the joint proaility of independent event, we multiply the individual proailitie. To determine the proaility of an event that can occur in two or more different way, we add the individual proailitie. Review Figure 10.9 The analyi of pedigree can trace Mendelian inheritance pattern in human. Review Figure 10.10, Allele and Their Interaction New allele arie y mutation, and many gene have multiple allele. Review Figure Dominance i ometime not complete, ince oth allele in a heterozygou organim may e expreed in the phenotype. Review Figure 10.13, 10.14

24 210 CHAPTER TEN Gene Interaction In epitai, the product of different gene interact to produce a phenotype. Review Figure Environmental variale uch a temperature, nutrition, and light affect gene action. In ome cae, the phenotype i the reult of the effect of everal gene and the environment, and inheritance i quantitative. Review Figure Gene and Chromoome Each chromoome carrie many gene. Gene located on the ame chromoome are aid to e linked, and they are often inherited together. Review Figure Linked gene can recomine y croing over in prophae I of meioi. The reult i recominant gamete, which have new comination of linked gene ecaue of the exchange. Review Figure 10.19, The ditance etween two gene on a chromoome i proportional to the frequency of croing over etween them. Genetic map are aed on recominant frequencie. Review Figure 10.21, ee We/CD Tutorial 10.2 ex Determination and ex-linked Inheritance ex chromoome carry gene that determine whether the organim will produce male or female gamete. The pecific function of X and Y chromoome differ among pecie. In fruit flie and mammal, the X chromoome carrie many gene, ut the Y chromoome ha only a few. Male have only one allele for X-linked gene, o rare allele how up phenotypically more often in male than in female. Review Figure 10.23, Non-Nuclear Inheritance Cytoplamic organelle uch a platid and mitochondria contain ome heritale gene. Cytoplamic organelle gene are generally inherited only from the mother ecaue male gamete contriute only their nucleu to the zygote at fertilization. ee We/CD Activitie 10.2 and 10.3 for a concept review of thi chapter. elf-quiz 1. In a imple Mendelian monohyrid cro, tall plant were croed with hort plant and the F 1 were croed among themelve. What fraction of the F 2 generation are oth tall and heterozygou? a c. 1 3 d. 2 3 e The phenotype of an individual a. depend at leat in part on the genotype.. i either homozygou or heterozygou. c. determine the genotype. d. i the genetic contitution of the organim. e. i either monohyrid or dihyrid. 3. The ABO lood group in human are determined y a multiple allelic ytem where I A and I B are codominant and dominant to I O. A neworn infant i type A. The mother i type O. Poile genotype of the father are: a. A, B or AB. A, B or O c. O only d. A or AB e. A or O 4. Which tatement aout an individual that i homozygou for an allele i not true? a. Each of it cell poee two copie of that allele.. Each of it gamete contain one copy of that allele. c. It i true-reeding with repect to that allele. d. It parent were necearily homozygou for that allele. e. It can pa that allele to it offpring. 5. Which tatement aout a tet cro i not true? a. It tet whether an unknown individual i homozygou or heterozygou.. The tet individual i croed with a homozygou receive individual. c. If the tet individual i heterozygou, the progeny will have a 1:1 ratio. d. If the tet individual i homozygou, the progeny will have a 3:1 ratio. e. Tet cro reult are conitent with Mendel model of inheritance. 6. Linked gene a. mut e immediately adjacent to one another on a chromoome.. have allele that aort independently of one another. c. never how croing over. d. are on the ame chromoome. e. alway have multiple allele. 7. In the F 2 generation of a dihyrid cro a. 4 phenotype appear in the ratio 9:3:3:1 if the loci are linked.. 4 phenotype appear in the ratio 9:3:3:1 if the loci are unlinked. c. 2 phenotype appear in the ratio 3:1 if the loci are unlinked. d. 3 phenotype appear in the ratio 1:2:1 if the loci are unlinked. e. 2 phenotype appear in the ratio 1:1 whether or not the loci are linked. 8. The ex of a human i determined y a. ploidy, the male eing haploid.. the Y chromoome. c. X and Y chromoome, the male eing XY. d. the numer of X chromoome, the male eing XO. e. Z and W chromoome, the male eing ZZ. 9. In epitai a. nothing change from generation to generation.. one gene alter the effect of another. c. a portion of a chromoome i deleted. d. a portion of a chromoome i inverted. e. the ehavior of two gene i entirely independent. 10. In human, potted teeth i caued y a dominant exlinked gene. A man with potted teeth whoe mother had normal teeth marrie a woman with normal teeth. Therefore, a. all of their daughter will have normal teeth.. all of their daughter will have potted teeth. c. all of their children will have potted teeth. d. half of their on will have potted teeth. e. none of their on will have potted teeth.

25 GENETIC: MENDEL AND BEYOND 211 Genetic Prolem 1. Uing the Punnett quare elow, how that for typical dominant and receive autoomal trait, it doe not matter which parent contriute the dominant allele and which the receive allele. Cro true-reeding tall plant (TT) with truereeding dwarf plant (tt). Female gamete Tall female Dwarf male Male gamete Tall male Female gamete Dwarf female Male gamete 2. The photograph how the hell of 15 ay callop, Argopecten irradian. Thee callop are hermaphroditic; that i, a ingle individual can reproduce exually, a did the pea plant of the F 1 generation in Mendel experiment. Three color cheme are evident: yellow, orange, and lack and white. The color-determining gene ha three allele. The top row how a yellow callop and a repreentative ample of it offpring, the middle row how a lack-and-white callop and it offpring, and the ottom row how an orange callop and it offpring. Aign a uitale ymol to each of the three allele participating in color control; then determine the genotype of each of the three parent individual and tell what you can aout the genotype of the different offpring. Explain your reult carefully. 4. A new tudent of genetic upect that a particular receive trait in fruit flie (dumpy wing, which are omewhat maller and more ell-haped than the wild-type) i ex-linked. A ingle mating etween a fly having dumpy wing (dp; female) and a fly with wild-type wing (Dp; male) produce 3 dumpy-winged female and 2 wild-type male. On the ai of thee data, i the trait ex-linked or autoomal? What were the genotype of the parent? Explain how thee concluion can e reached on the ai of o few data. 5. The ex of fihe i determined y the ame X-Y ytem a in human. An allele of one locu on the Y chromoome of the fih Leite caue a pigmented pot to appear on the doral fin. A male fih that ha a potted doral fin i mated with a female fih that ha an unpotted fin. Decrie the phenotype of the F 1 and the F 2 generation from thi cro. 6. In Droophila melanogater, the receive allele p, when homozygou, determine pink eye. Pp or PP reult in wildtype eye color. Another gene, on another chromoome, ha a receive allele, w, that produce hort wing when homozygou. Conider a cro etween female of genotype PPww and male of genotype ppww. Decrie the phenotype and genotype of the F 1 generation and of the F 2 generation produced y allowing the F 1 progeny to mate with one another. 7. On the ame chromoome of Droophila melanogater that carrie the p (pink eye) locu, there i another locu that affect the wing. Homozygou receive, yy, have litery wing, while the dominant allele By produce wild-type wing. The P and By loci are very cloe together on the chromoome; that i, the two loci are tightly linked. In anwering thee quetion, aume that no croing over occur. a. For the cro PPByBy ppyy, give the phenotype and genotype of the F 1 and of the F 2 generation produced y interreeding of the F 1 progeny.. For the cro PPyy ppbyby, give the phenotype and genotype of the F 1 and of the F 2 generation. c. For the cro of Quetion 7, what further phenotype() would appear in the F 2 generation if croing over occurred? d. Draw a nucleu undergoing meioi, at the tage in which the croing over (Quetion 7c) occurred. In which generation (P, F 1, or F 2 ) did thi croing over take place? 8. Conider the following cro of Droophila melanogater (allele a decried in Quetion 6): Male with genotype Ppww are croed with female of genotype ppww. Decrie the phenotype and genotype of the F 1 generation. 9. In the Andaluian fowl, a ingle pair of allele control the color of the feather. Three color are oerved: lue, lack, and plahed white. Croe among thee three type yield the following reult: PARENT PROGENY 3. how diagrammatically what occur when the F 1 offpring of the cro in Quetion 1 elf-pollinate. Female gamete Male gamete Black lue Blue and lack (1:1) Black plahed white Blue Blue plahed white Blue and plahed white (1:1) Black lack Black plahed white plahed white plahed white a. What progeny would reult from the cro lue lue?. If you want to ell egg, all of which would yield lue fowl, how hould you proceed?