Incomplete Dominance

• Neither allele is dominant• Heterozygotes are a blend

of homozygous phenotypes = no distinct expression of either allele

• F2 phenotypic and genotypic ratios are 1:2:1

• Snapdragon/Four o’clock flower color

• Eggplant color• Tay-Sachs disease

Try these• In a plant species, if the B allele (blue

flowers) and the W allele (white flowers) are incompletely dominant (b is light blue), what offspring ratio is expected in a cross between a blue-flowered plant and a white-flowered plant?

• What would be the phenotypic ratio of the flowers produced by a cross between two light blue flowers?

Codominance• Both alleles are

equally dominant• Heterozygotes

express both alleles = distinct expression of the gene products of both alleles can be detected

• MN blood group• F2 genotype and

phenotype ratios are 1:2:1

Genotype Phenotype

LMLM M

LMLN MN

LNLN N

Try These1. Cattle can be red (RR = all red hairs), white

(WW = all white hairs), or roan (RW = red & white hairs together.

a. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a roan bull.

b. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?

2. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together).

a. What pattern of inheritance does this illustrate? b. What percent of kittens would have tan fur if a

tabby cat is crossed with a black cat?