Topics to be covered:

1

• Topics to be covered
• codominant alleles,
• lethal alleles,
• multiple alleles,
• penetrance and expressivity

2
Codominant Alleles

• Heterozygote genotype gives rise to phenotype
  distinctly different from either of the
  homozygous genotypes.
• Example Blood types
• LMLM M type
• LMLN MN type
• LNLN N type

3

• Yellow coat color in guinea pigs CYCY
• Cream color CYCW
• White CWCW
• What genotypic and phenotypic ratios are matings
  between cream-colored individuals likely to
  produce?
• 1/4 CYCY yellow
• 1/2 CYCW cream
• 1/4 CWCW white

4
Lethal Alleles

• Genes that cause death before the individual can
  leave progeny.
• Can be dominant (rare to observe) or recessive.
• Suppose you cross two individuals and get a 21
  ratio rather than the expected 31 ratio. Why
  might this suggest a lethal gene being involved?

5

• Pelger anomaly in rabbits Pelgers are
  heterozygous (Pp) and have abnormal blood cells,
  normal individuals are homozygous (PP). The
  homozygous recessive genotypes (pp) have grossly
  deformed skeletons and usually die before or soon
  after birth. If Pelgers are mated together, what
  phenotypic ratio is expected in the adult F2?
• 1/2 Pelger 1/2 Normal

6
Multiple Alleles

• When more than 2 alleles are identified at a gene
  locus, then we have a multiple allelic series.
  The dominance hierarchy is given
  wgtwcogtwblgtwegtwch
• What would be the expected phenotypes for the
  following
• w/we
• wbl/wch
• we/we

7

• Self-incompatibility alleles in clover growth of
  pollen tube inhibited when style has the same
  allele. What genotypic ratios would be expected
  in the embryos of seeds from the following
  crosses?
• a. Seed parent S1S4 Pollen parent S3S4
• b. Seed parent S1S2 Pollen parent S1S2

8
Expressivity
9
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