Introduction Chapter Experimental genetics began in an abbey garden. 9.2 Experimental genetics began in an abbey garden

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1 Introduction Chapter 9 Dogs are one of man s longest genetic experiments. Over thousands of years, humans have chosen and mated dogs with specific traits. Resulting in a diverse array of dogs with distinct body types and behavioral traits. 202 Pearson Education, Inc. 9.2 Experimental genetics began in an abbey garden Heredity is the transmission of traits from one generation to the next. Genetics is the scientific study of heredity. Gregor Mendel began the field of genetics in the 860s, deduced the principles of genetics by breeding garden peas, and relied upon a background of mathematics, physics, and chemistry. 202 Pearson Education, Inc. 9.2 Experimental genetics began in an abbey garden In 866, Mendel correctly argued that parents pass on to their offspring discrete heritable factors and stressed that the heritable factors (today called genes), retain their individuality generation after generation. A heritable feature that varies among individuals, such as flower color, is called a character. Each variant for a character, such as purple or white flowers, is a trait. 202 Pearson Education, Inc.

2 Figure 9.2A 9.2 Experimental genetics began in an abbey garden True-breeding - offspring all identical to the parent. Hybrid - offspring of two different varieties P generation - True-breeding parents. F generation - Hybrid offspring of P generation F 2 generation - A cross of two F plants. 202 Pearson Education, Inc. Figure 9.2B Petal Carpel Stamen 2

3 Figure 9.2C_s3 Removal of stamens White Parents (P) Carpel Stamens 2 Transfer Purple of pollen 3 Carpel matures into pea pod Seeds from pod planted Offspring (F ) Figure 9.2D Character Dominant Traits Recessive Flower color Purple White Flower position Axial Terminal Seed color Yellow Green Seed shape Round Wrinkled Pod shape Inflated Constricted Pod color Green Yellow Stem length Tall Dwarf 9.3 Mendel s law of segregation describes the inheritance of a single character Monohybrid cross - cross between 2 individuals differing in a single character. Mendel performed a monohybrid cross between a plant with purple flowers and a plant with white flowers. The F generation produced all plants with purple flowers. A cross of F plants with each other produced an F 2 generation with ¾ purple and ¼ white flowers. 202 Pearson Education, Inc. 3

4 Figure 9.3A_s3 The Experiment P generation (true-breeding parents) Purple flowers White flowers F generation F 2 generation All plants have purple flowers Fertilization among F plants (F F ) 3 of plants have purple flowers of plants have white flowers 9.3 Mendel s law of segregation describes the inheritance of a single character The all-purple F generation did not produce light purple flowers, as predicted by the blending hypothesis. Mendel needed to explain why white color seemed to disappear in the F generation and white color reappeared in one quarter of the F 2 offspring. Mendel observed the same patterns of inheritance for six other pea plant characters. 202 Pearson Education, Inc. 9.3 Mendel s law of segregation describes the inheritance of a single character Mendel developed four hypotheses, described below using modern terminology.. Alleles are alternative versions of genes that account for variations in inherited characters. 2. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different. A homozygous genotype has identical alleles. A heterozygous genotype has two different alleles. 202 Pearson Education, Inc.

5 9.3 Mendel s law of segregation describes the inheritance of a single character 3. If the alleles of an inherited pair differ, then one determines the organism s appearance and is called the dominant allele. The other has no noticeable effect on the organism s appearance and is called the recessive allele. The phenotype is the appearance or expression of a trait. The genotype is the genetic makeup of a trait. The same phenotype may be determined by more than one genotype. 202 Pearson Education, Inc. 9.3 Mendel s law of segregation describes the inheritance of a single character. A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation. Mendel s hypotheses also explain the 3: ratio in the F 2 generation. The F hybrids all have a Pp genotype. A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine. 202 Pearson Education, Inc. Figure 9.3B_s3 The Explanation P generation Genetic makeup (alleles) Purple flowers White flowers PP pp Gametes All P All p F generation (hybrids) F 2 generation Gametes Fertilization 2 P All Pp Alleles segregate 2 Sperm from F plant P p p Phenotypic ratio 3 purple : white Genotypic ratio PP : 2 Pp : pp P Eggs from F plant p PP Pp Pp pp 5

6 Figure 9.3B_ F 2 generation Sperm from F plant P p Phenotypic ratio 3 purple : white Genotypic ratio PP : 2 Pp : pp P Eggs from F plant p PP Pp Pp pp 9. Homologous chromosomes bear the alleles for each character A locus (plural, loci) is the specific location of a gene along a chromosome. For a pair of homologous chromosomes, alleles of a gene reside at the same locus. Homozygous individuals have the same allele on both homologues. Heterozygous individuals have a different allele on each homologue. 202 Pearson Education, Inc. Figure 9. P Gene loci a B Dominant allele Homologous chromosomes P Genotype: PP aa Bb Homozygous Homozygous for the for the dominant recessive allele allele a b Recessive allele Heterozygous, with one dominant and one recessive allele 6

7 9.5 The law of independent assortment is revealed by tracking two characters at once A dihybrid cross is a mating of parental varieties that differ in two characters. Mendel performed the following dihybrid cross with the following results: P generation: round yellow seeds wrinkled green seeds F generation: all plants with round yellow seeds F 2 generation: 9/6 had round yellow seeds 3/6 had wrinkled yellow seeds 3/6 had round green seeds /6 had wrinkled green seeds 202 Pearson Education, Inc. Figure 9.5A P generation RRYY rryy Gametes RY ry F generation Sperm 2 RY ry 2 2 RY F 2 generation Eggs ry 2 The hypothesis of dependent assortment Data did not support; hypothesis refuted Sperm RrYy RY ry Ry ry RY RRYY RrYY RRYy RrYy ry 9 6 Eggs RrYY rryy RrYy rryy 3 Ry 6 RRYy RrYy RRyy Rryy 3 6 ry RrYy rryy Rryy rryy 6 The hypothesis of independent assortment Actual results; hypothesis supported Yellow round Green round Yellow wrinkled Green wrinkled Figure 9.5A_ P generation RRYY rryy Gametes RY ry F generation RrYy 7

8 Figure 9.5A_2 F generation RrYy Sperm 2 RY 2 ry F 2 generation 2 RY Eggs 2 ry The hypothesis of dependent assortment Data did not support; hypothesis refuted Figure 9.5A_3 F generation RrYy RY Sperm ry Ry ry RY RRYY RrYY RRYy RrYy Eggs ry Ry ry RrYY rryy RrYy rryy RRYy RrYy RRyy Rryy RrYy rryy Rryy rryy The hypothesis of independent assortment Actual results; hypothesis supported Yellow round Green round Yellow wrinkled Green wrinkled 9.5 The law of independent assortment is revealed by tracking two characters at once The following figure demonstrates the law of independent assortment as it applies to two characters in Labrador retrievers: black versus chocolate color, normal vision versus progressive retinal atrophy. 202 Pearson Education, Inc. 8

9 Figure 9.5B Blind Blind Phenotypes Black coat, Black coat, Chocolate coat, Chocolate coat, normal vision blind (PRA) normal vision blind (PRA) Genotypes B_N_ B_nn bbn_ bbnn Mating of double heterozygotes (black coat, normal vision) BbNn BbNn Blind Blind Phenotypic ratio of the offspring 9 Black coat, normal vision 3 Black coat, blind (PRA) 3 Chocolate coat, normal vision Chocolate coat, blind (PRA) Figure 9.6 What is the genotype of the black dog? Testcross Genotypes B_? bb Two possibilities for the black dog: BB or Bb Gametes B B b b Bb b Bb bb Offspring All black black : chocolate 9.5 The law of independent assortment is revealed by tracking two characters at once Mendel needed to explain why the F 2 offspring had new nonparental combinations of traits and a 9:3:3: phenotypic ratio. Mendel suggested that the inheritance of one character has no effect on the inheritance of another, suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and called this the law of independent assortment. 202 Pearson Education, Inc. 9

10 9.6 Geneticists can use the testcross to determine unknown genotypes A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual. A testcross can show whether the unknown genotype includes a recessive allele. Mendel used testcrosses to verify that he had truebreeding genotypes. The following figure demonstrates how a testcross can be performed to determine the genotype of a Lab with normal eyes. 202 Pearson Education, Inc. 9.7 Mendel s laws reflect the rules of probability Using his strong background in mathematics, Mendel knew that the rules of mathematical probability affected the segregation of allele pairs during gamete formation and the re-forming of pairs at fertilization. The probability scale ranges from 0 to. An event that is certain has a probability of and certain not to occur has a probability of Pearson Education, Inc. 9.7 Mendel s laws reflect the rules of probability The probability of a specific event is the number of ways that event can occur out of the total possible outcomes. Determining the probability of two independent events uses the rule of multiplication, in which the probability is the product of the probabilities for each event. The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities, called the rule of addition. 202 Pearson Education, Inc. 0

11 Figure 9.7 F genotypes Bb female Bb male Formation of eggs Formation of sperm 2 B ( 2 2 ) 2 Sperm b F 2 genotypes 2 B B B B Eggs b 2 b b B b b 9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees In a simple dominant-recessive inheritance of dominant allele A and recessive allele a, a recessive phenotype always results from a homozygous recessive genotype (aa) but a dominant phenotype can result from either the homozygous dominant genotype (AA) or a heterozygous genotype (Aa). Wild-type traits, those prevailing in nature, are not necessarily specified by dominant alleles. 202 Pearson Education, Inc. Figure 9.8A Dominant Traits Recessive Traits Freckles No freckles Widow s peak Straight hairline Free earlobe Attached earlobe

12 9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees The inheritance of human traits follows Mendel s laws. A pedigree shows the inheritance of a trait in a family through multiple generations, demonstrates dominant or recessive inheritance, and can also be used to deduce genotypes of family members. 202 Pearson Education, Inc. Figure 9.8B First generation (grandparents) Ff Ff ff Ff Second generation (parents, aunts, and uncles) FF or Ff Third generation (two sisters) Female Male Attached Free ff ff Ff Ff ff ff FF or Ff 9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene Inherited human disorders show either. recessive inheritance in which two recessive alleles are needed to show disease, heterozygous parents are carriers of the disease-causing allele, and the probability of inheritance increases with inbreeding, mating between close relatives. 2. dominant inheritance in which one dominant allele is needed to show disease and dominant lethal alleles are usually eliminated from the population. 202 Pearson Education, Inc. 2

13 Figure 9.9A Parents Normal Dd Normal Dd D Sperm d D DD Normal Dd Normal (carrier) Offspring Eggs d Dd Normal (carrier) dd Deaf 9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene The most common fatal genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is recessive and carried by about in 3 Americans. Dominant human disorders include achondroplasia, resulting in dwarfism, and Huntington s disease, a degenerative disorder of the nervous system. 202 Pearson Education, Inc. Table 9.9 3

14 Figure 9.9B 9.0 CONNECTION: New technologies can provide insight into one s genetic legacy New technologies offer ways to obtain genetic information before conception, during pregnancy, and after birth. Genetic testing can identify potential parents who are heterozygous carriers for certain diseases. 202 Pearson Education, Inc. 9.0 CONNECTION: New technologies can provide insight into one s genetic legacy Several technologies can be used for detecting genetic conditions in a fetus. Amniocentesis extracts samples of amniotic fluid containing fetal cells and permits karyotyping and biochemical tests on cultured fetal cells to detect other conditions, such as Tay-Sachs disease. Chorionic villus sampling removes a sample of chorionic villus tissue from the placenta and permits similar karyotyping and biochemical tests. Video: Ultrasound of Human Fetus 202 Pearson Education, Inc.

15 Figure 9.0A Ultrasound transducer Amniocentesis Amniotic fluid extracted Chorionic Villus Sampling (CVS) Tissue extracted Ultrasound from the transducer chorionic villi Fetus Placenta Uterus Cervix Fetus Placenta Chorionic villi Cervix Centrifugation Uterus Amniotic fluid Fetal cells Cultured cells Several hours Several weeks Biochemical and genetics tests Fetal cells Several hours Several weeks Several hours Karyotyping 9.0 CONNECTION: New technologies can provide insight into one s genetic legacy Blood tests on the mother at 20 weeks of pregnancy can help identify fetuses at risk for certain birth defects. Fetal imaging enables a physician to examine a fetus directly for anatomical deformities. The most common procedure is ultrasound imaging, using sound waves to produce a picture of the fetus. Newborn screening can detect diseases that can be prevented by special care and precautions. 202 Pearson Education, Inc. Figure 9.0B 5

16 9.0 CONNECTION: New technologies can provide insight into one s genetic legacy New technologies raise ethical considerations that include the confidentiality and potential use of results of genetic testing, time and financial costs, and determining what, if anything, should be done as a result of the testing. 202 Pearson Education, Inc. 9. Incomplete dominance results in intermediate phenotypes Mendel s pea crosses always looked like one of the parental varieties, called complete dominance. For some characters, the appearance of F hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance, in which neither allele is dominant over the other and expression of both alleles occurs. 202 Pearson Education, Inc. Figure 9.A P generation Red RR Gametes R r White rr F generation Pink hybrid Rr Gametes R 2 2 r F 2 generation 2 Sperm R 2 r Eggs R 2 2 r RR Rr rr rr 6

17 9. Incomplete dominance results in intermediate phenotypes Incomplete dominance does not support the blending hypothesis because the original parental phenotypes reappear in the F 2 generation. One example of incomplete dominance in humans is hypercholesterolemia, in which dangerously high levels of cholesterol occur in the blood and heterozygotes have intermediately high cholesterol levels. 202 Pearson Education, Inc. Figure 9.B LDL receptor HH Homozygous for ability to make LDL receptors LDL Genotypes Hh hh Heterozygous Homozygous for inability to make LDL receptors Phenotypes Cell Normal Mild disease Severe disease 9.2 Many genes have more than two alleles in the population Although an individual can at most carry two different alleles for a particular gene, more than two alleles often exist in the wider population. Human ABO blood group phenotypes involve three alleles for a single gene. The four human blood groups, A, B, AB, and O, result from combinations of these three alleles. The A and B alleles are both expressed in heterozygous individuals, a condition known as codominance. 202 Pearson Education, Inc. 7

18 9.2 Many genes have more than two alleles in the population In codominance, neither allele is dominant over the other and expression of both alleles is observed as a distinct phenotype in the heterozygous individual. AB blood type is an example of codominance. 202 Pearson Education, Inc. Figure 9.2_ Blood Group (Phenotype) Genotypes Carbohydrates Present on Red Blood Cells A I A I A or I A i Carbohydrate A B I B I B or I B i Carbohydrate B AB I A I B Carbohydrate A and Carbohydrate B O ii Neither Figure 9.2_2 Blood Group (Phenotype) Antibodies Present in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB A Anti-B B Anti-A AB None O Anti-A Anti-B 8

19 9.3 A single gene may affect many phenotypic characters Pleiotropy occurs when one gene influences many characteristics. Sickle-cell disease is a human example of pleiotropy. This disease affects the type of hemoglobin produced and the shape of red blood cells and causes anemia and organ damage. Sickle-cell and nonsickle alleles are codominant. Carriers of sickle-cell disease are resistant to malaria. 202 Pearson Education, Inc. Figure 9.3A Figure 9.3B An individual homozygous for the sickle-cell allele Produces sickle-cell (abnormal) hemoglobin The abnormal hemoglobin crystallizes, causing red blood cells to become sickle-shaped Sickled cell The multiple effects of sickled cells Damage to organs Kidney failure Heart failure Spleen damage Brain damage (impaired mental function, paralysis) Other effects Pain and fever Joint problems Physical weakness Anemia Pneumonia and other infections 9

20 9. A single character may be influenced by many genes Many characteristics result from polygenic inheritance, in which a single phenotypic character results from the additive effects of two or more genes. Human skin color is an example of polygenic inheritance. 202 Pearson Education, Inc. Figure 9. P generation aabbcc AABBCC (very light) (very dark) F generation AaBbCc AaBbCc Sperm F 2 generation Eggs Fraction of population Skin color 9.5 The environment affects many characters Many characters result from a combination of heredity and the environment. For example, skin color is affected by exposure to sunlight, susceptibility to diseases, such as cancer, has hereditary and environmental components, and identical twins show some differences. Only genetic influences are inherited. 202 Pearson Education, Inc. 20

21 Figure 9.5A Figure 9.5B 9.6 Chromosome behavior accounts for Mendel s laws The chromosome theory of inheritance states that genes occupy specific loci (positions) on chromosomes and chromosomes undergo segregation and independent assortment during meiosis. 202 Pearson Education, Inc. 2

22 9.6 Chromosome behavior accounts for Mendel s laws Mendel s laws correlate with chromosome separation in meiosis. The law of segregation depends on separation of homologous chromosomes in anaphase I. The law of independent assortment depends on alternative orientations of chromosomes in metaphase I. 202 Pearson Education, Inc. Figure 9.6_s3 F generation R r y All yellow round seeds (RrYy) Y R r r R Y y Metaphase I Y of meiosis y R r r R Y y Anaphase I Y y R r Metaphase II r R Y y Y y Gametes Y Y y y Y Y y y R R r r r r R R RY ry ry Ry Fertilization F 2 generation 9 :3 :3 : Figure 9.6_ Sperm RY ry Ry ry RY RRYY RrYY RRYy RrYy ry RrYY rryy RrYy rryy Eggs Ry RRYy RrYy RRyy Rryy ry RrYy rryy Rryy rryy 9 6 Yellow round 3 6 Green round 3 6 Yellow wrinkled 6 Green wrinkled 22

23 9.7 SCIENTIFIC DISCOVERY: Genes on the same chromosome tend to be inherited together Bateson and Punnett studied plants that did not show a 9:3:3: ratio in the F 2 generation. What they found was an example of linked genes, which are located close together on the same chromosome and tend to be inherited together. 202 Pearson Education, Inc. Figure 9.7_ The Experiment Purple flower PpLl PpLl Long pollen Phenotypes Purple long Purple round Red long Red round Observed offspring Prediction (9:3:3:) Figure 9.7_2 The Explanation: Linked Genes Parental diploid cell PpLl P L p l Meiosis Most gametes P L p l Fertilization P L PL Most P L offspring Eggs p l pl P L Sperm PL pl P L p l p l p l 3 purple long : red round Not accounted for: purple round and red long 23

24 9.8 SCIENTIFIC DISCOVERY: Crossing over produces new combinations of alleles Crossing over between homologous chromosomes produces new combinations of alleles in gametes. Linked alleles can be separated by crossing over, forming recombinant gametes. The percentage of recombinants is the recombination frequency. 202 Pearson Education, Inc. Figure 9.8A p L p l P L Parental gametes p l Tetrad (pair of homologous chromosomes) Crossing over p L P l Recombinant gametes Figure 9.8C The Experiment The Explanation Gray body, long wings (wild type) Black body, vestigial wings GgLl Female G L g l g l g l ggll Male GgLl ggll Crossing over Female Male G L g l G l gl g l Offspring Eggs Sperm Gray long Black vestigial Gray vestigial Black long Offspring G L g l G l g L g l g l g l g l Parental Recombinant Parental phenotypes Recombinant phenotypes 39 recombinants Recombination frequency 0.7 or 7% 2,300 total offspring 2

25 Figure 9.8C_ The Experiment Gray body, long wings (wild type) GgLl Female Black body, vestigial wings ggll Male Offspring: Gray long Black vestigial Gray vestigial Black long Parental phenotypes Recombinant phenotypes 39 recombinants Recombination frequency 0.7 or 7% 2,300 total offspring Figure 9.8C_2 The Explanation GgLl Female G L g l g l g l ggll Male Crossing over G L g l G l g L g l Eggs Sperm G L g l Offspring g l G l g l g l g L g l Parental Recombinant 9.9 Geneticists use crossover data to map genes When examining recombinant frequency, Morgan and his students found that the greater the distance between two genes on a chromosome, the more points there are between them where crossing over can occur. Recombination frequencies can thus be used to map the relative position of genes on chromosomes. 202 Pearson Education, Inc. 25

26 Figure 9.9A Section of chromosome carrying linked genes g c l 7% 9% 9.5% Recombination frequencies Figure 9.9B Short aristae Mutant phenotypes Black body (g) Cinnabar eyes (c) Vestigial wings (l) Brown eyes Long aristae (appendages on head) Gray body (G) Red eyes (C) Wild-type phenotypes Normal wings (L) Red eyes 9.20 Chromosomes determine sex in many species Many animals have a pair of sex chromosomes, designated X and Y, that determine an individual s sex. In mammals, males have XY sex chromosomes, females have XX sex chromosomes, the Y chromosome has genes for the development of testes, and an absence of the Y allows ovaries to develop. 202 Pearson Education, Inc. 26

27 Figure 9.20A X Y Figure 9.20B Parents (diploid) Male XY Female XX Gametes (haploid) 22 X Sperm 22 Y 22 X Egg Offspring (diploid) XX Female XY Male 9.20 Chromosomes determine sex in many species Grasshoppers, roaches, and some other insects have an X-O system, in which O stands for the absence of a sex chromosome, females are XX, and males are XO. In certain fishes, butterflies, and birds, the sex chromosomes are Z and W, males are ZZ, and females are ZW. 202 Pearson Education, Inc. 27

28 Figure 9.20C Male 22 X Female 22 XX Figure 9.20D Male 76 ZZ Female 76 ZW 9.20 Chromosomes determine sex in many species Some organisms lack sex chromosomes altogether. In bees, sex is determined by chromosome number. Females are diploid. Males are haploid. 202 Pearson Education, Inc. 28

29 Figure 9.20E Male Female Chromosomes determine sex in many species In some animals, environmental temperature determines the sex. For some species of reptiles, the temperature at which the eggs are incubated during a specific period of development determines whether the embryo will develop into a male or female. Global climate change may therefore impact the sex ratio of such species. 202 Pearson Education, Inc. 9.2 Sex-linked genes exhibit a unique pattern of inheritance Sex-linked genes are located on either of the sex chromosomes. The X chromosome carries many genes unrelated to sex. The inheritance of white eye color in the fruit fly illustrates an X-linked recessive trait. 202 Pearson Education, Inc. 29

30 Figure 9.2A Figure 9.2B Female Male X R X R X r Y X r Sperm Y Eggs X R X R X r X R Y R red-eye allele r white-eye allele Figure 9.2C Female Male X R X r X R Y Sperm Y x R Eggs X R X r X R X R X r X R X R Y X r Y R red-eye allele r white-eye allele 30

31 Figure 9.2D Female Male X R X r X r Y X r Sperm Y Eggs X R X r X R X r X r X r X R Y X r Y R red-eye allele r white-eye allele 9.22 CONNECTION: Human sex-linked disorders affect mostly males Most sex-linked human disorders are due to recessive alleles and seen mostly in males. A male receiving a single X-linked recessive allele from his mother will have the disorder. A female must receive the allele from both parents to be affected. 202 Pearson Education, Inc CONNECTION: Human sex-linked disorders affect mostly males Recessive and sex-linked human disorders include hemophilia, characterized by excessive bleeding because hemophiliacs lack one or more of the proteins required for blood clotting, red-green color blindness, a malfunction of lightsensitive cells in the eyes, and Duchenne muscular dystrophy, a condition characterized by a progressive weakening of the muscles and loss of coordination. 202 Pearson Education, Inc. 3

32 Figure 9.22 Queen Victoria Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis Female Male Hemophilia Carrier Normal 9.23 EVOLUTION CONNECTION: The Y chromosome provides clues about human male evolution The Y chromosome provides clues about human male evolution because Y chromosomes are passed intact from father to son and mutations in Y chromosomes can reveal data about recent shared ancestry. 202 Pearson Education, Inc. Figure

33 You should now be able to. Describe pangenesis theory and the blending hypothesis. Explain why both ideas are now rejected. 2. Define and distinguish between true-breeding organisms, hybrids, the P generation, the F generation, and the F 2 generation. 3. Define and distinguish between the following pairs of terms: homozygous and heterozygous; dominant allele and recessive allele; genotype and phenotype. Also, define a monohybrid cross and a Punnett square. 202 Pearson Education, Inc. You should now be able to. Explain how Mendel s law of segregation describes the inheritance of a single characteristic. 5. Describe the genetic relationships between homologous chromosomes. 6. Explain how Mendel s law of independent assortment applies to a dihybrid cross. 7. Explain how and when the rule of multiplication and the rule of addition can be used to determine the probability of an event. 202 Pearson Education, Inc. You should now be able to 8. Explain how family pedigrees can help determine the inheritance of many human traits. 9. Explain how recessive and dominant disorders are inherited. Provide examples of each. 0. Compare the health risks, advantages, and disadvantages of the following forms of fetal testing: amniocentesis, chorionic villus sampling, and ultrasound imaging. 202 Pearson Education, Inc. 33

34 You should now be able to. Describe the inheritance patterns of incomplete dominance, multiple alleles, codominance, pleiotropy, and polygenic inheritance. 2. Explain how the sickle-cell allele can be adaptive. 3. Explain why human skin coloration is not sufficiently explained by polygenic inheritance.. Define the chromosome theory of inheritance. Explain the chromosomal basis of the laws of segregation and independent assortment. 202 Pearson Education, Inc. You should now be able to 5. Explain how linked genes are inherited differently from nonlinked genes. 6. Describe T. H. Morgan s studies of crossing over in fruit flies. Explain how Sturtevant created linkage maps. 7. Explain how sex is genetically determined in humans and the significance of the SRY gene. 8. Describe patterns of sex-linked inheritance and examples of sex-linked disorders. 9. Explain how the Y chromosome can be used to trace human ancestry. 202 Pearson Education, Inc. Figure 9.UN0 Homologous Alleles, residing chromosomes at the same locus Fertilization Meiosis Gamete from the other Paired alleles, parent different forms of a gene Haploid gametes (allele pairs separated) Diploid zygote (containing paired alleles) 3

35 Figure 9.UN02 Incomplete dominance Red RR White rr Pink Rr Figure 9.UN03 Single gene Pleiotropy Multiple characters Figure 9.UN0 Multiple genes Polygenic inheritance Single characters (such as skin color) 35

36 Figure 9.UN05 Genes chromosomes located on alternative versions called (a) at specific locations called if both are the same, the genotype is called if different, the genotype is called (b) (c) heterozygous the expressed allele is called the unexpressed allele is called (d) (e) inheritance when the phenotype is in between is called (f) Figure 9.UN06 36

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