Coat Color in Mice
2 different genes determine only 3
different phenotypes, rather than 4
phenotypes typical of a dihybrid cross
Homozygous, recessive genotype
at C-locus is epistatic to genotype
at B-locus
Another epistasis example flower color in peas
• Flower color is determined by two different genes
• The pigment in colored flowers is produced by a
two-step process
Genotype
C_P_
C_pp
ccP_
ccpp
Flower color
Enzyme
activities
Flowers colored; Functional
anthocyanin
enzymes from
produced
both genes
Flowers white;
p enzyme not
no anthocynain functional
produced
Flowers white;
c enzyme not
no anthocynain functional
produced
Flowers white;
P and c enzyme
no anthocynain not functional
produced
The result is therefore a ratio of 9
flowered plants: 7 white plants
Pleiotropic genes
Yellow and gray coat color in
mice
• In 1904, researchers begin with a truebreeding strain of gray mice crossed with
yellow mice
• The F1 generation was 50% gray and 50%
yellow
– Yellow must be dominant to gray
– The yellow mice must have been heterozygotes
Yellow and gray coat color
• Next a cross of two yellow mice was made
– One predicts a 3:1 ratio of yellow to gray mice
– The result was a 2:1 ratio of yellow to gray
mice
The ratio of 2:1 suggests a lethal
gene
• In the heterozygous condition, the Y allele
causes a yellowing of the coat
• In the homozygous condition, the Y alleles
produce enough gene product to cause the
mouse to die
• The Y allele is said to be pleiotropic; it
affects more than one phenotypic
characteristic
Punnett Square predictions
Male
Female
Y
y
Y
YY
Yy
y
Yy
yy
Phenylketonuria - another
example of pleiotropy
• Metabolic defect caused by homozygous recessive
alleles for enzyme phenylalanine hydroxylase
Phenylketonuria - another
example of pleiotropy
• Primary effect of mutant gene is to cause
toxic substances to build up in the brain,
leading to mental impairment
• The mutant gene also affects:
– the synthesis of melanin pigment, resulting in
PKU patients having light brown or blond hair
– Posture
– Organ function
Figure 10.18a
Fruit color is highly variable in bell peppers.
Figure 10.18b
Crosses between pure lines produce novel colors.
Parental
generation
X
Yellow
Brown
F1 generation
Red
Self-fertilization
F2 generation
Red
9/16
Yellow
3/16
Brown
3/16
Green
1/16
Figure 10.18c
Model to explain 9 : 3 : 3 : 1 pattern observed above: Two genes interact to produce pepper color.
Genotype
Color
Explanation of color
R-Y-
Red
Red pigment + no chlorophyll
rrY-
Yellow
Yellow pigment + no chlorophyll
R-yy
Brown
Red pigment + chlorophyll
rryy
Green
Yellow pigment + chlorophyll
Gene 1
Gene 2
R = Red
Y = Absence of green (no chlorophyll)
r = Yellow
y = Presence of green (+ chlorophyll)
(-) = R or r
(-) = Y or y
Skin color
in corn
snakes
Gene interactions in corn snakes
• Two loci
– One allele causes black pigment to be deposited
(dominant allele is B+ and recessive is b)
– One allele causes orange pigment to be
deposited (dominant allele is O + and recessive
is o)