Coat Color in Mice

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Coat Color in Mice
2 different genes determine only 3 different
phenotypes, rather than 4 phenotypes typical of a
dihybrid cross
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Homozygous, recessive genotype at C-locus is
epistatic to genotype at B-locus
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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

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Genotype Flower color Enzyme activities
C_P_ Flowers colored anthocyanin produced Functional enzymes from both genes
C_pp Flowers white no anthocynain produced p enzyme not functional
ccP_ Flowers white no anthocynain produced c enzyme not functional
ccpp Flowers white no anthocynain produced P and c enzyme not functional
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The result is therefore a ratio of 9 flowered
plants 7 white plants
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Pleiotropic genes
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Yellow and gray coat color in mice

• In 1904, researchers begin with a true-breeding
  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

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Yellow and gray coat color

• Next a cross of two yellow mice was made
• One predicts a 31 ratio of yellow to gray mice
• The result was a 21 ratio of yellow to gray mice

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The ratio of 21 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

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Punnett Square predictions
Male Female Y y
Y YY Yy
y Yy yy
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Phenylketonuria - another example of pleiotropy

• Metabolic defect caused by homozygous recessive
  alleles for enzyme phenylalanine hydroxylase

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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

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Figure 10.18a
Fruit color is highly variable in bell peppers.
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Figure 10.18b
Crosses between pure lines produce novel colors.
Parental generation
X
Yellow
Brown
F1 generation
Red
Self-fertilization
F2 generation
Red
Yellow
Brown
Green
9/16
3/16
3/16
1/16
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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
Y Absence of green (no chlorophyll)
R Red
y Presence of green ( chlorophyll)
r Yellow
(-) Y or y
(-) R or r
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Skin color in corn snakes
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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)