Exercise 1: Incomplete Dominance in
Snapdragons
In some cases, neither allele is
dominant over the other allele, this is an example of
incomplete dominance. In this case,
individuals heterozygous for the trait would exhibit an
intermediate phenotype. Thus, the genotypic ratio is the same as
the phenotypic ratio in cases of incomplete dominance.
For example, in snapdragons, red flowered plants (RR) and white
flowered plants (rr) are both homozygous. If a red snapdragon is
crossed with a white snapdragon, all of the offspring have pink
flowers (Rr) because the genes that code for the production of red
and white pigments (R and r, respectively) are both expressed. Work
through the following problems:
1. In a
cross between two pink snapdragons, what are the expected
phenotypic and genotypic ratios of the offspring?
2. In a
cross between a white and pink snapdragon what are the expected
phenotypic and genotypic ratios of the offspring?
3. In a
cross between a red and a pink snapdragon, what are the expected
phenotypic and genotypic ratios of the offspring?