Dihybrid Cross Problem Set
Problem 13: AaBb dihybrid cross involving epistasis
Tutorial to help answer the question
Two unlinked loci effect mouse hair color. AA or Aa mice are agouti. Mice with genotype aa are albino because all pigment production is blocked, regardless of the phenotype at the second locus. At the second locus, the B allele (agouti coat) is dominant to the b allele (black coat). What would be the result of a cross between two agouti mice of genotype AaBb?
A Punnet Square for the AaBb x AaBb cross is shown below. Rather than the
9:3:3:1 seqregation of phenotypes normally seen with an AaBb x AaBb
dihybrid cross, the phenotypic ratio is 9:3:4. There are only three
different phenotypes for hair color, agouti, black and albino. The
individual with genotype recessive for both traits, i.e. aabb, has the same
albino phenotype as the aaBB and aaBb individuals due to epistasis. The
albino phenotype masks any phenotype that might be caused by the recessive
homozygous, recessive bb genotype.
The Biology Project
University of Arizona
Monday, March 30, 1998
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