Mendelian Genetics

1
Mendelian Genetics

• Simple Probabilities a Little Luck

2
Genetics

• the study of heredity its mechanisms
• Gregor Mendel
• reported experimental results in 1865/66
• rediscovered in 1903 by de Vries, Correns von
  Tschermak

3
Genetics

• Before Mendel, heredity was seen as
• the blending of parental contributions
• unpredictable
• Mendel demonstrated that heredity
• involves distinct particles
• is statistically predictable

4
Cross pollinationFigure 10.1
5
Mendels Experiments

• the model system
• garden pea varieties
• easy to grow
• short generation time
• many offspring
• bisexual
• reciprocal cross-pollination
• self-compatible
• self-pollination

6
Mendels 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
7 characters, 14 traitsTable 10.1
8
one of Mendels charactersFigure 10.2
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Mendels Experiments

• Mendels 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

10
Mendels Experiments

• Mendels 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

11
Mendels Experiments

• Mendels experimental design
• Protocol 1 monohybrid crosses
• Results
• all F1 progeny exhibited the same trait
• F2 progeny exhibited both parental traits in a
  31 ratio (F1 trait alternate trait)

12
Mendels Experiments

• Mendels 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 31 in F2

13
seven traits were inherited similarlyTable 10.1
14
Mendels interpretationinheritance does not
involve blendingFigure 10.3
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Mendels Experiments

• Mendels 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
• gtMendels particles are genes lt

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Mendels Experiments

• Mendels 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

17
recessive traits disappear in the F1
generationFigure 10.4
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Mendels Experiments

• Mendels 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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Mendels Experiments

• Mendels 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

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Punnett to the rescueFigure 10.4
21
P (SS or ss) p(S)1 x
p(s)1F1 (Ss) p(Ss) 1 x 11
p(S)1/2, p(s)1/2, so F2 p(SS)
1/2 x 1/21/4 p(ss) 1/2 x 1/21/4
p(Ss)1/2x1/21/4 x 21/2
22
Punnett explained by meiosisFigure 10.5
F1 Ss replication S-S s-s anaphase I S-S or
s-s anaphase II S or S or s or s
23
Mendels Experiments

• Mendels 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
• F1 1/2 Round, 1/2 wrinkled
• F2 3/4 Round, 1/4 wrinkled OR all wrinkled

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Mendels Experiments

• Mendels 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)

25
a test cross distinguishes between a homozygous
dominant and a heterozygous parentFigure 10.6
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Mendels Experiments

• Mendels 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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Mendels 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
  91 ratio
• non-parental pairs of traits appeared in 3/8 of
  F2 at a 11 ratio

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combining probabilities of two charactersFigure
10.7
29
four different gametes by meiosis in F1
dihybrid progenyFigure 10.8
or
30
Mendels 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
  91 ratio
• nonparental pairs of traits appeared in 3/8 of F2
  at a 11 ratio
• phenotypic ratios 9331

31
Mendels Experiments

• Protocol 2 dihybrid crosses
• phenotypic ratios 9331
• predictable if alleles assort independently
• character A - 31 dominantrecessive
• character B - 31 dominantrecessive
• 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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Mendels Experiments

• Protocol 2 dihybrid crosses
• a dihybrid test cross (A_B_ x aabb)
• F1 all with dominant parent phenotype, or
• 1111 phenotypes

33
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

34
a pedigree of Huntingtons diseaseFigure 10.10
35
a pedigree of albinismFigure 10.11
36
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
phenotypesFigure 10.13
39
some Mendelian luck

• Incomplete Dominance
• alters creates new intermediate phenotypes
• reveals genotypes
• Co-dominance
• creates new dominant phenotypes

40
co-dominance produces additional
phenotypesFigure 10.14
41
some Mendelian luck

• genes may interact
• epistasis
• for mouse coat color
• BB or Bb gt agouti, bb gt black
• AA or Aa gt colored, aa gt white
• AaBb x AaBb gt 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 maizeFigure 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

46
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

47
variation in heterozygotes due to differences
in penetrance expressivityvariation in
the population due to differences in penetrance,
expressivity genotypeFigure 10.17
48
Drosophila melanogasterFigure 10.18
49
More Mendelian luck gene linkage

• gene linkage was first demonstrated in Drosophila
  melanogaster
• some genes do not assort independently
• F2 phenotype ratios are not 9331
• F1 test cross ratios are not 1111
• more parental combinations appear than are
  expected
• fewer recombinant combinations appear than are
  expected

50
Mendels luck some genes are linkedFigure 10.18
2300 test cross progeny
51
hypotheticalreproduction without crossing over
at prophase I of meiosis
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crossing over can change allele combinations of
linked lociFigure 10.19
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recombination frequency depends on
distanceFigure 10.20
391/23000.17 17 map units
54
More Mendelian luck gene linkage

• if genes were completely linked, only parental
  phenotypes would result
• if genes assort independently phenotypes arise in
  9331 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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Mendels 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 Xautosomes determines sex
• birds, moths butterflies ZZ male, ZW female

57
Mendels 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

58
sex-linked genesFigure 10.23
59
Mendels luck sex-linked genes

• human sex-linked inheritance can be deduced from
  pedigree analysis

60
inheritance of X-linked geneFigure 10.24
61
Mendels 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