Figure 10.20a
Kernel color is quantitative trait.
Parental
generation
X
F2 generation
Number of each
phenotype
F1 generation
20
15
6
15
6
1
1
Phenotypes
Figure 10.20b
X
aa bb cc
(pure line white)
Model to explain
inheritance
of kernel color
AA BB CC
(pure line red)
Aa Bb Cc
(medium red)
Self-fertilization
20
15
6
1 aabbcc
2 Aabbcc
2 aaBbcc
2 aabbCc
0
1
1
1 AAbbcc
4 AaBbcc
1 aaBBcc
4 AabbCc
1 aabbCC
4 aaBbCc
2
2 AABbcc
2 AAbbCc
2 AaBBcc
2 AabbCC
2 aaBBCc
2 aaBbCC
8 AaBbCc
3
15
1 AABBcc
4 AABbCc
1 AAbbCC
4 AaBbCC
1 aaBBCC
4 AaBBCc
4
6
2 AABBCc
2 AaBBCC
2 AABbCC
5
Number of red pigment alleles (A, B, or C) in genotype
1
1 AABBCC
6
Predict the phenotype frequencies
of a cross between AaBb x Aabb

Assume the A locus and B locus are on
different chromosomes
Two genes, each with two alleles, are known to
influence coat color in Labrador retrievers. Let’s
call the alleles for the first gene B and b and E and e
for the second gene. The ratio of colors is 9 black: 3
chocolate: 4 yellow.
1. Suggest a mechanism for inheritance of coat color
2. Write out all possible genotypes and describe the
phenotype of each genotype.
Penetrance

The percentage of individuals with a
given genotype who exhibit the
phenotype associated with that
genotype
Pedigree showing incomplete
penetrance
Reasons for incomplete
penetrance

Epistatic genes, pleiotropic genes or
other genes which suppress the
expression of the genotype
Expressivity

The degree to which a genotype is
expressed in the phenotype
Expressivity in
beagles
Each of these dogs has
the dominant allele for
piebald (black and
white) spotting
 The degree of spotting
varies among
individuals

Polydactyly
Penetrance and expressivity
Environmental
effects on fur
color in
Himalayan
rabbits
Fur color
in Siamese
cats