mendelian genetics simple probabilities & a little luck
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Mendelian Genetics
Simple Probabilities & a Little Luck
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Genetics
•the study of heredity & its mechanisms
•Gregor Mendel–reported experimental results in 1865/66
–rediscovered in 1903 by de Vries, Correns & von Tschermak
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Genetics
•Before Mendel, heredity was seen as–the blending of parental contributions
–unpredictable•Mendel demonstrated that heredity –involves distinct particles–is statistically predictable
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Cross pollinationFigure 10.1
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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
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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
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7 characters,
14 traitsTable 10.1
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one of Mendel’s charactersFigure 10.2
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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
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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
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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)
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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
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seven traits were inherited similarly
Table 10.1
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Mendel’s interpretation:
inheritance does
not involve blendingFigure 10.3
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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 <=
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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
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recessive traits
disappear in the
F1 generation
Figure 10.4
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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]
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Mendel’s Experiments•Mendel’s experimental design–Protocol #1: monohybrid crosses•symbolic representationP: SS x ssF1: Ss gamete formation S or sself pollination: S with S
s with sS with s or
s with SF2: SS, ss, Ss, sS
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Punnett to the rescueFigure 10.4
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P: (SS or ss) p(S)=1 x p(s)=1
F1: (Ss) 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
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Punnett explaine
d by
meiosisFigure 10.5
F1: Ssreplicat
ion
S-S & s-s
anaphase I
S-S or s-s
anaphase II
S or S or s or
s
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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 wrinkledF1: 1/2 Round, 1/2 wrinkled
F2: 3/4 Round, 1/4 wrinkled OR all wrinkled
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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)F1: 1/2 Round (Rr), 1/2 wrinkled (rr)
F2: 3/4 Round, 1/4 wrinkled OR all wrinkled
(RR,Rr,rR,rr) (rr)
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a test cross
distinguishes
between a homozygou
s dominant
and a heterozyg
ous parentFigure 10.6
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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
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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
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combining probabilities of
two character
sFigure 10.7
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four differe
nt gametes
by meiosis
in F1
dihybrid
progenyFigure 10.8
or
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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
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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
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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
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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
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a pedigree of Huntington’s
diseaseFigure 10.10
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a pedigree of albinism
Figure 10.11
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some Mendelian luck
•Multiple alleles–a single gene may have more than two alleles and multiple phenotypes
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One Character, Four Alleles, Five PhenotypesFigure 10.12
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incomplete dominance:
intermediate phenotypes
Figure 10.13
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some Mendelian luck
•Incomplete Dominance –alters creates new intermediate phenotypes
–reveals genotypes•Co-dominance–creates new dominant phenotypes
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co-dominance produces additional phenotypes
Figure 10.14
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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
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white, black & agouti Figure
10.15
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some Mendelian luck•genes may interact–hybrid vigor (heterosis)•hybrids are more vigorous than either inbred parent
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hybrid vigor in maize
Figure 10.16
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some Mendelian luck•genes may interact–quantitative traits•some traits are determined by many genes, each of which may have many alleles
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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
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variation in heterozygotes
due to differences in penetrance & expressivity
variation in the population due to
differences in penetrance,
expressivity & genotype
Figure 10.17
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Drosophila melanogasterFigure 10.18
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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
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2300testcrossprogeny
Mendel’s luck: some genes are linked
Figure 10.18
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hypothetical
reproduction
without crossing over at prophase
I of meiosis
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crossing over can change allele combinations of linked loci
Figure 10.19
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recombination frequency depends on distanceFigure 10.20
391/2300=0.17
17 map units
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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)
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chromosome mappingFigure 10.21
YyMm x yymm wt yell. min. y/m expected/1000 250 250 250 250 actual/1000 323 178 177 322
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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
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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
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sex-linked genesFigure 10.23
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Mendel’s luck: sex-linked genes
•human sex-linked inheritance can be deduced from pedigree analysis
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inheritance of X-linked geneFigure 10.24
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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