CH 11: Mendel / The Gene. Concept 11.1: Mendel used the scientific approach to identify two laws of inheritance

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1 CH 11: Mendel / The Gene What genetic pinciples account fo the passing of taits fom paents to offsping? The blending hypothesis is the idea that genetic mateial fom the two paents blends togethe (the way blue and yellow paint blend to make geen) The paticulate hypothesis is the idea that paents pass on discete heitable units (genes) Mendel documented a paticulate mechanism though his expeiments with gaden peas Concept 11.1: Mendel used the scientific aoach to identify two laws of inheitance Figue 11.2 Technique 1 Mendel discoveed the basic pinciples of heedity by beeding gaden peas in caefully planned expeiments Mendel pobably chose to wok with peas because Thee ae many vaieties with distinct heitable featues, o chaactes (such as flowe colo); chaacte vaiants (such as puple o white flowes) ae called taits He could contol mating between plants He tended the gaden? aental geneation () 3 esults Fist filial geneation offsping (F 1 ) 2 Stamens Capel 4 5 Mendel tacked only chaactes that occued in two distinct altenative foms He used vaieties that wee tue-beeding (plants that poduce offsping of the same vaiety when they self-pollinate) In a typical expeiment, Mendel mated two contasting, tue-beeding vaieties, a pocess called hybidization The tue-beeding paents ae the geneation The hybid offsping of the geneation ae called the F 1 geneation When F 1 individuals self-pollinate o coss- pollinate with othe F 1 hybids, the F 2 geneation is poduced Figue Expeiment Geneation (tue-beeding paents) F 1 Geneation (hybids) F 2 Geneation The Law of Segegation uple flowes White flowes All plants had puple flowes Self- o coss-pollination 705 puple-floweed plants 224 white-floweed plants 1

2 Table 11.1 Mendel easoned that in the F 1 plants, the heitable facto fo white flowes was hidden o masked in the pesence of the puple-flowe facto He called the puple flowe colo a dominant tait and the white flowe colo a ecessive tait The facto fo white flowes was not diluted o destoyed because it eaeaed in the F 2 geneation Mendel obseved the same patten of inheitance in six othe pea plant chaactes, each epesented by two taits What Mendel called a heitable facto is what we now call a gene Mendel s Model Mendel developed a model to explain the 3:1 inheitance patten he obseved in F 2 offsping Fou concepts make up this model Fist, altenative vesions of genes account fo vaiations in inheited chaactes Fo example, the gene fo flowe colo in pea plants exists in two vesions, one fo puple flowes and the othe fo white flowes These altenative vesions of a gene ae now called alleles Each gene esides at a specific locus on a specific chomosome Figue 11.4 Each gene esides at a specific locus on a specific chomosome Locus fo flowe-colo gene Allele fo puple flowes Allele fo white flowes ai of homologous chomosomes Second, fo each chaacte, an oganism inheits two alleles, one fom each paent Mendel made this deduction without knowing about the existence of chomosomes Two alleles at a paticula locus may be identical, as in the tue-beeding plants of Mendel s geneation Altenatively, the two alleles at a locus may diffe, as in the F 1 hybids Thid, if the two alleles at a locus diffe, then one (the dominant allele) detemines the oganism s aeaance, and the othe (the ecessive allele) has no noticeable effect on aeaance In the flowe-colo example, the F 1 plants had puple flowes because the allele fo that tait is dominant 2

3 Figue Geneation Fouth (now known as the law of segegation), the two alleles fo a heitable chaacte sepaate (segegate) duing gamete fomation and end up in diffeent gametes Thus, an egg o a spem gets only one of the two alleles that ae pesent in the oganism This segegation of alleles coesponds to the distibution of homologous chomosomes to diffeent gametes in meiosis Aeaance: uple flowes Genetic makeup: Gametes: F 1 Geneation Aeaance: Genetic makeup: Gametes: F 2 Geneation fom F 1 () plant p uple flowes fom F 1 () plant White flowes p p p 3 : 1 Useful Genetic Vocabulay Mendel s segegation model accounts fo the 3:1 atio he obseved in the F 2 geneation of his numeous cosses ossible combinations of spem and egg can be shown using a unnett squae A capital lette epesents a dominant allele, and a lowecase lette epesents a ecessive allele Fo example, is the puple-flowe allele and p is the white-flowe allele An oganism with two identical alleles fo a chaacte is said to be homozygous fo the gene contolling that chaacte An oganism that has two diffeent alleles fo a gene is said to be heteozygous fo the gene contolling that chaacte Unlike homozygotes, heteozygotes ae not tuebeeding Figue 11.6 henotype Genotype An oganism s taits do not always eveal its genetic composition uple (homozygous) 1 Theefoe, we distinguish between an oganism s phenotype, o physical aeaance, and its genotype, o genetic makeup In the example of flowe colo in pea plants, and plants have the same phenotype (puple) but diffeent genotypes 3 uple uple (heteozygous) (heteozygous) 2 1 White (homozygous) 1 atio 3:1 atio 1:2:1 3

4 The Testcoss Figue 11.7 Technique How can we tell the genotype of an individual with the dominant phenotype? Such an individual could be eithe homozygous dominant o heteozygous The answe is to cay out a testcoss: beeding the mystey individual with a homozygous ecessive individual If any offsping display the ecessive phenotype, the mystey paent must be heteozygous esults Dominant phenotype, unknown genotype: o? edictions If puple-floweed paent is p p All offsping puple o o ecessive phenotype, known genotype: p If puple-floweed paent is p p offsping puple and offsping white The Law of Independent Assotment Figue 11.8 Expeiment Geneation YY yy Mendel identified his second law of inheitance by following two chaactes at the same time Cossing two tue-beeding paents diffeing in two chaactes poduces dihybids in the F 1 geneation, heteozygous fo both chaactes A dihybid coss, a coss between F 1 dihybids, can detemine whethe two chaactes ae tansmitted to offsping as a package o independently Each gene (pai of alleles) assots independently of each othe gene (pai of alleles) duing gamete fomation This law alies to genes on diffeent, nonhomologous chomosomes o those fa apat on the same chomosome - BUT Genes located nea each othe on the same chomosome tend to be inheited togethe F 1 Geneation edictions edicted offsping in F 2 geneation esults Y y Gametes Y Hypothesis of dependent assotment Y y ¾ YY Yy Yy yy o henotypic atio 3: y Yy Hypothesis of independent assotment Y Y y y Y y Y y YY YY Yy Yy YY YY Yy Yy Yy Yy yy yy Yy Yy yy yy henotypic atio 9:3:3:1 henotypic atio aoximately 9:3:3:1 Figue 11.8b edicted offsping in F 2 geneation Hypothesis of dependent assotment y Y y Y YY Yy ¾ Yy yy henotypic atio 3:1 Y Hypothesis of independent assotment y y Y Y y y Y YY YY Yy Yy YY YY Yy Yy Yy Yy yy yy Yy Yy yy yy Concept 11.2: The laws of pobability goven Mendelian inheitance Mendel s laws of segegation and independent assotment eflect the ules of pobability When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss In the same way, the alleles of one gene segegate into gametes independently of anothe gene s alleles esults henotypic atio 9:3:3:1 henotypic atio aoximately 9:3:3:1 4

5 The Multiplication and Addition ules Alied to Monohybid Cosses Figue 11.9 Segegation of alleles into eggs Segegation of alleles into spem The multiplication ule states that the pobability that two o moe independent events will occu togethe is the poduct of thei individual pobabilities This can be alied to an F 1 monohybid coss Segegation in a heteozygous plant is like fliing a coin: Each gamete has a 1 2 chance of caying the dominant allele and a 1 2 chance of caying the ecessive allele We can aly the ules of pobability to pedict the outcome of cosses involving multiple chaactes The addition ule states that the pobability that any one of two o moe mutually exclusive events will occu is calculated by adding togethe thei individual pobabilities Example: If we coss F 1 heteozygotes of genotype Yy, we can calculate the pobability of diffeent genotypes among the F 2 geneation It can be used to figue out the pobability that an F 2 plant fom a monohybid coss will be heteozygous athe than homozygous Aa x Aa 25% Aa and 25% aa = 50% Aa Figue 11.UN02 Example: In the coss Yy yy, we can calculate the pobability of offsping showing at least two ecessive taits Concept 11.3: Inheitance pattens ae often moe complex than pedicted by simple Mendelian genetics Not all heitable chaactes ae detemined as simply as the taits Mendel studied Howeve, the basic pinciples of segegation and independent assotment aly even to moe complex pattens of inheitance Deviation include When alleles ae not completely dominant o ecessive When a gene has moe than two alleles When a single gene influences multiple phenotypes 5

6 Degees of Dominance Complete dominance occus when phenotypes of the heteozygote and dominant homozygote ae identical In incomplete dominance, the phenotype of F 1 hybids is somewhee between the phenotypes of the two paental vaieties In codominance, two dominant alleles affect the phenotype in sepaate, distinguishable ways Figue Geneation F 1 Geneation ed C C Gametes Gametes F 2 Geneation C C W C C C C C W C W C C C W C W C C W C W C W ink C C W White C W C W Fo any chaacte, dominant/ecessive elationships of alleles depend on the level at which we examine the phenotype Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the bain At the oganismal level, the allele is ecessive At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant At the molecula level, the alleles ae codominant Fequency of Dominant Alleles Dominant alleles ae not necessaily moe common in populations than ecessive alleles Fo example, one baby out of 400 in the United States is bon with exta finges o toes, a dominant tait called polydactyly Multiple Alleles Figue (a) The thee alleles fo the ABO blood goups and thei cabohydates Many genes exist in populations in moe than two allelic foms Fo example, the fou phenotypes of the ABO blood goup in humans ae detemined by thee alleles of the gene: I A, I B, and i. The enzyme (I) adds specific cabohydates to the suface of blood cells The enzyme encoded by I A adds the A cabohydate, and the enzyme encoded by I B adds the B cabohydate; the enzyme encoded by the i allele adds neithe Allele Cabohydate (b) Blood goup genotypes and phenotypes Genotype ed blood cell aeaance henotype (blood goup) A I A I A I A o I A i A B I B I B o I B i B I B I A I B AB i none ii O 6

7 leiotopy Most genes have multiple phenotypic effects, a popety called pleiotopy Fo example, pleiotopic alleles ae esponsible fo the multiple symptoms of cetain heeditay diseases, such as cystic fibosis and sickle-cell disease Extending Mendelian Genetics Two o Moe Genes In epistasis, a gene at one locus altes the phenotypic expession of a gene at a second locus Fo example, in Labado etieves and many othe mammals, coat colo depends on two genes One gene detemines the pigment colo (with alleles B fo black and b fo bown) The othe gene (with alleles E fo colo and e fo no colo) detemines whethe the pigment will be deposited in the hai Figue BbEe BbEe olygenic Inheitance BE be Be BBEE BE be BbEE Be BBEe be BbEe BbEE bbee BbEe bbee BBEe BbEe BBee Bbee Quantitative chaactes ae those that vay in the population along a continuum Quantitative vaiation usually indicates polygenic inheitance, an additive effect of two o moe genes on a single phenotype Skin colo in humans is an example of polygenic inheitance be BbEe bbee Bbee bbee 9 : 3 : 4 Figue AaBbCc AaBbCc henotypes: Numbe of dak-skin alleles: Natue and Nutue: The Envionmental Impact on henotype Depatues fom Mendelian genetics aise when the phenotype fo a chaacte depends on envionment as well as genotype The nom of eaction is the phenotypic ange of a genotype influenced by the envionment The phenotypic ange is geneally boadest fo polygenic chaactes Such chaactes ae called multifactoial because genetic and envionmental factos collectively influence phenotype 7

8 Concept 11.4: Many human taits follow Mendelian pattens of inheitance Humans ae not good subjects fo genetic eseach Geneation time is too long aents poduce elatively few offsping Beeding expeiments ae unacceptable Howeve, basic Mendelian genetics endues as the foundation of human genetics edigee Analysis A pedigee is a family tee that descibes the inteelationships of paents and childen acoss geneations Inheitance pattens of paticula taits can be taced and descibed using pedigees edigees can also be used to make pedictions about futue offsping We can use the multiplication and addition ules to pedict the pobability of specific phenotypes Figue Key Male 1st geneation (gandpaents) 3d geneation (two sistes) Female Bonus: Find evidence, eseach studies, fo eithe of these Ww ww Widow s peak Affected male WW o Ww ww 2nd geneation (paents, aunts, and uncles) Ww ww ww Ww Ww ww Affected female Ww ww No widow s peak (a) Is a widow s peak a dominant o ecessive tait? Mating FF o ff Attached ealobe ff ff Offsping, in bith ode (fist-bon on left) ff FF o ff Fee ealobe (b) Is an attached ealobe a dominant o ecessive tait? ecessively Inheited Disodes ecessively inheited disodes show up only in individuals homozygous fo the allele Caies ae heteozygous individuals who cay the ecessive allele but ae phenotypically nomal A a Aa A AA Aa (caie) aents Aa a Aa (caie) aa Albino Cystic Fibosis If a ecessive allele that causes a disease is ae, then the chance of two caies meeting and mating is low Consanguineous (between close elatives) matings incease the chance of mating between two caies of the same ae allele Most societies and cultues have laws o taboos against maiages between close elatives Cystic fibosis is the most common lethal genetic disease in the United States,stiking one out of evey 2,500 people of Euopean descent The cystic fibosis allele esults in defective o absent chloide tanspot channels in plasma membanes leading to a buildup of chloide ions outside the cell Symptoms include mucus buildup in some intenal ogans and abnomal absoption of nutients in the small intestine 8

9 Sickle-Cell Disease: A Genetic Disode with Evolutionay Implications Sickle-cell disease affects one out of 400 Afican- Ameicans The disease is caused by the substitution of a single amino acid in the hemoglobin potein in ed blood cells In homozygous individuals, all hemoglobin is abnomal (sickle-cell) Symptoms include physical weakness, pain, ogan damage, and even paalysis Heteozygotes (said to have sickle-cell tait) ae usually healthy but may suffe some symptoms About one out of ten Afican-Ameicans has sickle-cell tait, an unusually high fequency of an allele with detimental effects in homozygotes Heteozygotes ae less susceptible to the malaia paasite, so thee is an advantage to being heteozygous Dominantly Inheited Disodes Figue Some human disodes ae caused by dominant alleles Dominant alleles that cause a lethal disease ae ae and aise by mutation Achondoplasia is a fom of dwafism caused by a ae dominant allele d Dwaf Dd D Dd Dwaf aents dd d dd d Dd Dwaf dd Multifactoial Disodes The timing of onset of a disease significantly affects its inheitance Huntington s disease is a degeneative disease of the nevous system The disease has no obvious phenotypic effects until the individual is about 35 to 45 yeas of age Once the deteioation of the nevous system begins the condition is ievesible and fatal Many diseases, such as heat disease, diabetes, alcoholism, mental illnesses, and cance, have both genetic and envionmental components Lifestyle has a temendous effect on phenotype fo cadiovascula health and othe multifactoial chaactes 9

10 Genetic Counseling Based on Mendelian Genetics SUMMAY elationship among alleles of a single gene Desciption Example Genetic counselos can povide infomation to pospective paents concened about a family histoy fo a specific disease Complete dominance of one allele Incomplete dominance of eithe allele Heteozygous phenotype same as that of homozygous dominant Heteozygous phenotype intemediate between the two homozygous phenotypes C C C C W C W C W Codominance Both phenotypes expessed in heteozygotes I A I B Multiple alleles In the whole population, some genes have moe than two alleles ABO blood goup alleles I A, I B, i leiotopy One gene is able to affect multiple phenotypic chaactes Sickle-cell disease Figue 11.UN06 elationship among two o moe genes Desciption Example Epistasis The phenotypic expession of one gene affects the expession of anothe gene BbEe BE BE be Be BbEe be be Be be 9 : 3 : 4 olygenic inheitance A single phenotypic chaacte is affected by two o moe genes AaBbCc AaBbCc 10