Recessive Epistasis in Collinsia
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]
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
[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]