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Mendelian Genetics Simple Probabilities & a Little Luck Genetics • the study of heredity & its mechanisms • Gregor Mendel – reported experimental results in 1865/66 – rediscovered in 1903 by de Vries, Correns & von Tschermak Genetics • Before Mendel, heredity was seen as – the blending of parental contributions – unpredictable • Mendel demonstrated that heredity – involves distinct particles – is statistically predictable Cross pollination Figure 10.1 Mendel’s Experiments • the model system – garden pea varieties • easy to grow • short generation time • many offspring • bisexual –reciprocal cross-pollination • self-compatible –self-pollination Mendel’s Experiments • garden pea varieties – many variable characters • a character is a heritable feature –flower color • a trait is a character state –blue flowers, white flowers, etc. • a heritable trait is reliably passed down • a true-breeding variety produces the same trait each generation 7 characters, 14 traits Table 10.1 one of Mendel’s characters Figure 10.2 Mendel’s Experiments • Mendel’s experimental design – selected 7 characters with distinct traits – crossed plants with one trait to plants with the alternate trait (P = “parental” generation) – self-pollinated offspring of P (F1 = first filial generation) – scored traits in F1 and F2 generations Mendel’s Experiments • Mendel’s experimental design – Protocol #1: monohybrid crosses • parents were true-breeding for alternate traits of one character • parents were reciprocally cross-pollinated • F1 progeny were self-pollinated • traits of F1 & F2 progeny were scored Mendel’s Experiments • Mendel’s experimental design – Protocol #1: monohybrid crosses – Results • all F1 progeny exhibited the same trait • F2 progeny exhibited both parental traits in a 3:1 ratio (F1 trait: alternate trait) Mendel’s Experiments • Mendel’s experimental design – Protocol #1: monohybrid crosses – Analysis • F1 trait is dominant • alternate trait is recessive –disappears from the F1 generation –reappears, unchanged, in F2 – Relevance • all seven characters have dominant and recessive traits appearing 3:1 in F2 seven traits were inherited similarly Table 10.1 Mendel’s interpretation: inheritance does not involve blending Figure 10.3 Mendel’s Experiments • Mendel’s experimental design – Protocol #1: monohybrid crosses – Interpretation • inheritance is by discrete units (particles) • hereditary particles occur in pairs • particles segregate at gamete formation • particles are unaffected by combination • =>Mendel’s particles are genes <= Mendel’s Experiments • Mendel’s experimental design – Protocol #1: monohybrid crosses • symbolic representation –P: SS x ss –F1: Ss • each parent packages one gene in each gamete • gametes combine randomly recessive traits disappear in the F1 generation Figure 10.4 Mendel’s Experiments • Mendel’s experimental design – Protocol #1: monohybrid crosses • [terminology –different versions of a gene = alleles –two copies of an allele = homozygous –one copy of each allele = heterozygous –genetic constitution = genotype –round or wrinkled seeds = phenotype –the genotype is not always seen in the phenotype] Mendel’s Experiments • Mendel’s experimental design – Protocol #1: monohybrid crosses • symbolic representation P: SS x ss F1: Ss gamete formation S or s self pollination: S with S s with s S with s or s with S F2: SS, ss, Ss, sS Punnett to the rescue Figure 10.4 P: (SS or ss) p(S)=1 F1: (Ss) x p(s)=1 p(Ss) =1 x 1=1 p(S)=1/2, p(s)=1/2, so F2: p(SS) =1/2 x 1/2=1/4 p(ss) =1/2 x 1/2=1/4 p(Ss)=[1/2x1/2=1/4] x 2=1/2 F1: Ss replication S-S & s-s anaphase I S-S or s-s anaphase II S or S or s or s Punnett explained by meiosis Figure 10.5 Mendel’s Experiments • Mendel’s experimental design – Protocol #1: monohybrid crosses • if you know the genotypes of the parental generation you can predict the phenotypes of the F1 & F2 generations P: Round x wrinkled F 1: 1/2 Round, 1/2 wrinkled F2: 3/4 Round, 1/4 wrinkled OR all wrinkled Mendel’s Experiments • Mendel’s experimental design – Protocol #1: monohybrid crosses • if you know the genotypes of the parental generation you can predict the phenotypes of the F1 & F2 generations P: Round (Rr) x wrinkled (rr) F 1: 1/2 Round (Rr), 1/2 wrinkled (rr) F2: 3/4 Round, 1/4 wrinkled OR all wrinkled (RR,Rr,rR,rr) (rr) a test cross distinguishes between a homozygous dominant and a heterozygous parent Figure 10.6 Mendel’s Experiments • Mendel’s experimental design – Protocol #2: dihybrid crosses • P: crossed true breeding plants with different traits for two characters • F1: scored phenotypes & self-pollinated • F2: scored phenotypes Mendel’s Experiments • Protocol #2: dihybrid crosses – results • F1: all shared the traits of one parent • F2: –traits of both parents occurred in 5/8 of F2 at a 9:1 ratio –non-parental pairs of traits appeared in 3/8 of F2 at a 1:1 ratio combining probabilities of two characters Figure 10.7 or four different gametes by meiosis in F1 dihybrid progeny Figure 10.8 Mendel’s Experiments • Protocol #2: dihybrid crosses – results • F1: all shared traits of one parent • F2: –traits of both parents occurred in 5/8 of F2 at a 9:1 ratio –nonparental pairs of traits appeared in 3/8 of F2 at a 1:1 ratio –phenotypic ratios: 9:3:3:1 Mendel’s Experiments • Protocol #2: dihybrid crosses – phenotypic ratios: 9:3:3:1 • predictable if alleles assort independently –character A - 3:1 dominant:recessive –character B - 3:1 dominant:recessive –characters A & B »9 dominant A & dominant B »3 dominant A & recessive B »3 recessive A & dominant B »1 recessive A & recessive B Mendel’s Experiments • Protocol #2: dihybrid crosses – a dihybrid test cross (A_B_ x aabb) • F1 all with dominant parent phenotype, or • 1:1:1:1 phenotypes Mendel without the experiments: pedigrees • tracking inheritance patterns in human populations – uncontrolled experimentally – small progenies – unknown parental genotypes • Mendelian principles can interpret phenotypic inheritance patterns a pedigree of Huntington’s disease Figure 10.10 a pedigree of albinism Figure 10.11 some Mendelian luck • Multiple alleles – a single gene may have more than two alleles and multiple phenotypes One Character, Four Alleles, Five Phenotypes Figure 10.12 incomplete dominance: intermediate phenotypes Figure 10.13 some Mendelian luck • Incomplete Dominance – alters creates new intermediate phenotypes – reveals genotypes • Co-dominance – creates new dominant phenotypes co-dominance produces additional phenotypes Figure 10.14 some Mendelian luck • genes may interact – epistasis • for mouse coat color –BB or Bb => agouti, bb => black –AA or Aa => colored, aa => white • AaBb x AaBb => 9 agouti, 3 black, 4 white – 9 AA or Aa with BB or Bb – 3 AA or Aa with bb – 3 aa with BB, Bb; 1 aa with bb = 4 white white, black & agouti Figure 10.15 some Mendelian luck • genes may interact – hybrid vigor (heterosis) • hybrids are more vigorous than either inbred parent hybrid vigor in maize Figure 10.16 some Mendelian luck • genes may interact – quantitative traits • some traits are determined by many genes, each of which may have many alleles some Mendelian luck • environment may alter phenotype – some traits are altered by the environment of the organism • penetrance: proportion of a population expressing the phenotype • expressivity: degree of expression of the phenotype variation in heterozygotes due to differences in penetrance & expressivity variation in the population due to differences in penetrance, expressivity & genotype Figure 10.17 Drosophila melanogaster Figure 10.18 More Mendelian luck: gene linkage • gene linkage was first demonstrated in Drosophila melanogaster – some genes do not assort independently • F2 phenotype ratios are not 9:3:3:1 • F1 test cross ratios are not 1:1:1:1 –more parental combinations appear than are expected –fewer recombinant combinations appear than are expected Mendel’s luck: some genes are linked Figure 10.18 2300 test cross progeny hypothetical reproduction without crossing over at prophase I of meiosis crossing over can change allele combinations of linked loci Figure 10.19 recombination frequency depends on distance Figure 10.20 391/2300=0.17 17 map units More Mendelian luck: gene linkage • if genes were completely linked, only parental phenotypes would result • if genes assort independently phenotypes arise in 9:3:3:1 ratio in F2 • when genes are linked, recombinant phenotypes are fewer than expected • recombinant frequencies depend on distance – distances can be estimated from recombination rates (1% = 1 map unit) chromosome mapping Figure 10.21 YyMm x yymm expected/1000 actual/1000 wt yell. min. y/m 250 250 250 250 323 178 177 322 Mendel’s luck: sex-linked genes • Sex determination – honey bees: diploid female, haploid male – grasshopper: XX female, XO male – mammals: XX female, XY male • SRY gene determines maleness – Drosophila: XX female, XY male • ratio of X:autosomes determines sex – birds, moths & butterflies: ZZ male, ZW female Mendel’s luck: sex-linked genes • genes carried on X chromosome are absent from the Y chromosome • a recessive sex-linked allele is expressed in the phenotype of a male – females may be “carriers” – males express the single allele sex-linked genes Figure 10.23 Mendel’s luck: sex-linked genes • human sex-linked inheritance can be deduced from pedigree analysis inheritance of X-linked gene Figure 10.24 Mendel’s Principles • Principle of segregation – two alleles for a character are not altered by time spent together in a diploid nucleus • Principle of independent assortment – segregation of alleles for one character does not affect segregation of alleles for another character • unless both reside on the same chromosome