Incomplete & Codominance

Complete dominance The traits studied in Mendel’s pea plants

were examples of inheritance (or simple dominance): One allele completely masks the other allele. This is not always the case.

We will compare the

heterozygous genotypes for

complete dominance to the heterozygous genotypes for two other types of inheritance.

Incomplete Dominance: One allele is not completely dominant over the

other. There is a “blending” of phenotypes when both alleles are present.

A monohybrid cross (of heterozygotes)results in three separate phenotype possibilities, instead of just two as in simple or complete dominance.

Example: Incomplete Dominance

The plant called the “four-o-clocks” (Mirabilis) inherits flower color through incompletely dominant alleles.

R = red color W = white color

A cross between a red and white flower- ing plant will yield a third phenotype —pink, in the F1 generation.

If two of the F1 off -spring are crossed, we don’t see the 3:1 F2 ratio as in Mendel’s crosses.

Genotypic Ratio 1RR : 2RW : 1WW

Phenotypic Ratio 1red:2 pink:1white

Genotypic & Phenotypic ratios are the same

CodominanceCodominance occurs when both alleles for the

trait are expressed simultaneously in the

heterozygous genotype.

Example: In horses and cattle, there is a coat that exists of red and white hairs. It is known as “roan.”

R may indicate Red hairs

W may indicate White hairs

An animal with roan coat color would have a

genotype of RW. Both alleles are expressed at the same time (if the animal is heterozygous).

(There are also a “blue roan” horse in which there are codominant black and white genes)