1. The Experiments of Gregor Mendel

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1 0/9/205 Chapte 4: Mendel and the Gene Idea. The Expeiments of Gego Mendel 2. Beyond Mendelian Genetics 3. Human Genetics. The Expeiments of Gego Mendel Chapte eading TECHNIQUE aental geneation () ESULTS Fist filial geneation offsping (F ) 2 Stamens Capel Gego Mendel Deduced the basics of inheitance in the 9 th centuy by analyzing pea plant cosses: examined seveal chaactes exhibiting dominant & ecessive taits o phenotypes

2 0/9/205 Mendel s Cosses ) Coss plants that beed tue fo chaacte of inteest EXEIMENT Geneation (tue-beeding paents) F Geneation (hybids) uple flowes White flowes All plants had puple flowes 2) Coss the esulting hybids (a monohybid coss) F 2 Geneation 705 puple-floweed plants 224 white-floweed plants esults fo Vaious Chaactes Fo each chaacte Mendel saw the same basic patten: a 3: atio of dominant to ecessive taits following a monohybid coss henotype vs Genotype henotype Genotype uple (homozygous) 3 uple uple (heteozygous) (heteozygous) 2 White (homozygous) atio 3: atio :2: 2

3 0/9/205 Geneation Aeaance: uple flowes White flowes Genetic makeup: Gametes: p F Geneation Aeaance: uple flowes Genetic makeup: Gametes: / 2 / 2 p F 2 Geneation p Mendel s Law of Segegation Each gamete ends up with only one of the two copies of each gene in the paent. e.g., eithe the puple flowe allele o the white flowe allele 50% of gametes contain one copy, 50% contain the othe p 3 This is due to the segegation of chomosomes duing meiosis. Test Cosses Test cosses ae used to eveal an unknown genotype: coss the unknown with a homozygous ecessive individual TECHNIQUE edictions Dominant phenotype, unknown genotype: o?? x ecessive phenotype, known genotype: If If o p p p p if all offsping display dominant phenotype: homozygous dominant () if ~½ each of ecessive and dominant phenotypes: heteozygous () ESULTS All offsping puple o p /2 offsping puple and /2 offsping white Genetic Alleles & Genetic Loci Genes can have diffeent vesions known as alleles. e.g., puple flowe vs white flowe alleles Allele fo puple flowes Locus fo flowe-colo gene Homologous pai of chomosomes Allele fo white flowes All genes have a specific chomosomal location efeed to as its locus. homologous chomosomes have the same genetic loci 3

4 0/9/205 Impotant Genetic Tems Chaacte physical chaacteistic (e.g., eye colo) henotype individual s obsevable taits (e.g., bown eyes) Allele diffeent foms o vesions of the same gene Genotype the alleles an individual has fo a given gene (e.g., AA o Aa o aa) Homozygous the 2 alleles fo a gene ae identical (AA o aa) Heteozygous the 2 alleles fo a gene ae diffeent (Aa) Dominant allele (designated by capital lette: A) copy of the allele detemines phenotype (A ) ecessive allele (designated by lowe-case lette: a) affects phenotype only when homozygous (aa) Homozygous dominant = AA Homozygous ecessive = aa Heteozygous = Aa All genetic inheitance is based on the pobability of inheiting specific genetic alleles fom each paent. Genetics & obability Segegation of alleles into eggs / 2 / 2 / 2 / 2 / 4 / 4 / 4 / 4 Segegation of alleles into spem 4

5 0/9/205 The Natue of obability The pobability of multiple events haening simultaneously is the poduct of the pobabilities of each single event. The pobability of one event O anothe is the sum of the pobabilities of each event. EXEIMENT Geneation F Geneation edictions edicted offsping of F 2 geneation /2 /2 Y y Multiple Gene Inheitance YY Gametes Y Hypothesis of dependent assotment /2 3 Y YY /2 y yy o henotypic atio 3: y yy Hypothesis of independent assotment Y Y y y Y YY YY Yy Y YY YY Yy y Yy yy yy y Yy yy yy In a dihybid coss, the pobability of each combined phenotype is the poduct of each individual phenotype. Expected atios: 9 Y 3 Y ESULTS /6 /6 /6 /6 henotypic atio 9:3:3: henotypic atio aoximately 9:3:3: 3 yy yy Mendel s Law of Independent Assotment Each pai of genetic alleles segegates independently of all othe pais of genetic alleles duing gamete fomation (meiosis). the distibution of the alleles of a given gene into gametes has no connection to the distibution of alleles fo othe genes alies only to genes on diffeent chomosomes 5

6 0/9/ Beyond Mendelian Genetics Chapte eading Geneation ed C C White C W C W Incomplete Dominance F Geneation Gametes C C W ink C C W Dominant allele is expessed to lesse degee when only copy is pesent (heteozygous): Gametes / C 2 / 2 C W F 2 Geneation / 2 C / C 2 / C W 2 C C C C W heteozygotes have an intemediate phenotype (e.g., pink vs ed o white) / 2 C W C C W C W C W Codominance Two diffeent alleles when paied togethe ae expessed equally (e.g., ABO blood type) A & B alleles ae codominant (both alleles expessed) Allele I A Cabohydate I B i none (a) The thee alleles fo the ABO blood goups and thei associated cabohydates Genotype A B ed blood cell aeaance henotype (blood goup) I A I A o I A i A I B I B o I B i I A I B B AB O allele is ecessive ii (b) Blood goup genotypes and phenotypes O 6

7 0/9/205 / 4 BC / 4 bc BbCc BbCc / 4 BC / 4 bc / 4 Bc / 4 bc BBCC BbCC BBCc BbCc BbCC bbcc BbCc bbcc Epistasis The expession of a genetic allele fom one locus altes the expession of alleles at anothe locus. / 4 Bc / 4 bc BBCc BbCc BBcc BbCc bbcc Bbcc 9 : 3 : 4 Bbcc bbcc in this example the homozygous ecessive genotype fo one gene oveides the expession of the othe gene olygenic Inheitance Many phenotypes ae due to the effects of multiple genes (such as skin colo). / 8 / 8 / 8 / 8 / 8 AaBbCc AaBbCc / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 in this example, 3 diffeent genes influence one phenotype esulting in 64 diffeent vaiants / 8 / 8 / 8 henotypes: / 64 6 / 64 5 / / 64 5 / 64 6 / 64 / 64 Numbe of dak-skin alleles: Human Genetics Chapte eading

8 0/9/205 Key Male Female st geneation (gandpaents) 2nd geneation (paents, aunts, and uncles) Affected male Affected female Ww ww Ww ww ww Ww ww Mating Offsping, in bith ode (fist-bon on left) Ww Ww ww edigees Diagams illustating family histoy that allow inheitance pattens to be deduced. 3d geneation (two sistes) Widow s peak WW o Ww ww (a) Is a widow s peak a dominant o ecessive tait? No widow s peak We can t do expeimental cosses on humans, we can only analyze existing family pedigees. A ecessive Human Tait st geneation (gandpaents) Ff Ff ff Ff 2nd geneation (paents, aunts, and uncles) FF o Ff ff ff Ff Ff ff 3d geneation (two sistes) ff FF o Ff Attached ealobe Fee ealobe (b) Is an attached ealobe a dominant o ecessive tait? Albinism is also ecessive aents Aa Aa A a A AA Aa (caie) a Aa (caie) aa Albino 8

9 0/9/205 Achondoplasia is Dominant aents Dwaf Dd dd D d d Dd Dwaf dd d Dd Dwaf dd Key Tems fo Chapte 4 tait, phenotype, genotype, allele, locus homozygous, heteozygous dominant, ecessive, codominant tue-beeding, monohybid, dihybid test coss, unnet squae epistasis polygenic inheitance pedigee elevant Chapte Questions -4 9