MGA2 14-13

Recessive Epistasis in Collinsia

    Flowers of the genus Collinsia (Plantaginaceae), known as Chinese Houses, are typically blue, but pink or white colour variants also occur in nature. Mendelian analysis of monohybrid and dihybrid crosses allows the genetic of colour to be determined.

 Enzyme 1 (from locus W) converts  a white precursor to a pink pigment, and Enzyme 2 (from locus M) converts the pink pigment to blue. One allele (w+ or m+) at either locus produces enough enzyme activity to catalyze each transformation, in either homozygous or heterozygous genotypes. Blockage of the second step (by an mm genotype) prevents conversion of the pink pigment to blue, and produces a pink flower. However, blockage of the first step by a ww genotype prevents formation of a pink pigment, irrespective of the genotype at locus M, and the flower remains white. That is, a recessive genotype at locus M suppresses expression of the dominant genotype at locus W.

Collinsia heterophylla
Collinsia heterophylla
[white phase photo © Joseph Dougherty, M.D./ecology.org; pink phase photo ©2001 Gary A. Monroe; purple phase photo by Dr. Robert T. & Margaret Orr © California Academy of Sciences]

Diagram ©2002 my WH Freeman; All text material ©2012 by Steven M. Carr