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MCAT Question A Day - 5/1/13 - Answer!

The gene A leads to the production of a blue plant pigment, while the gene B leads to the deposition of that blue pigment in the petals of the flower. If a dihybrid cross of two heterozygotes (AaBb x AaBb) produces some offspring with blue petals and others with white petals in a phenotypic ratio of 9:7, what genetic phenomenon is the most likely explanation?

A.  Pleiotropy
B.  Linkage
C.  Epistasis
D.  Polygenic inheritance

The correct answer is (C). Epistasis refers to a kind of gene interaction in which alleles at one locus can influence the effects of alleles at a separate locus. In this case, all AB individuals can make and deposit the blue pigment in their petals and will be blue (9), all aa individuals would not be able to produce the pigment and would be white (3), all A_bb individuals would be unable to deposit the pigment in their petals and would be white (3), and all aabb individuals would neither be able to make nor deposit the pigment and would be white (1).