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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 pollinationFigure 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 traitsTable 10.1
one of Mendel’s charactersFigure 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 blendingFigure 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 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
Punnett to the rescueFigure 10.4
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
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
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
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)
a test cross
distinguishes
between a homozygou
s dominant
and a heterozyg
ous parentFigure 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 character
sFigure 10.7
four differe
nt gametes
by meiosis
in F1
dihybrid
progenyFigure 10.8
or
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
diseaseFigure 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 PhenotypesFigure 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 melanogasterFigure 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
2300testcrossprogeny
Mendel’s luck: some genes are linked
Figure 10.18
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 distanceFigure 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 mappingFigure 10.21
YyMm x yymm wt yell. min. y/m expected/1000 250 250 250 250 actual/1000 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 genesFigure 10.23
Mendel’s luck: sex-linked genes
•human sex-linked inheritance can be deduced from pedigree analysis
inheritance of X-linked geneFigure 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
