Incomplete Dominance & Codominance

Not all traits are controlled by single genes with simple dominant - recessive alleles:

Multiple alleles
Polygenic traits
Incomplete dominance

Multiple alleles = trait that is determined by more than two alleles (ex. Human blood types)

Polygenic traits = trait influenced by several genes; genes may be on same chromosome or on different one (ex. Human eye color, weight, skin tone)

Incomplete dominance = one allele is not completely dominant over the other (blend) heterozygous condition somewhere in between (flowers…red, white, pink)

Codominance= condition in which both alleles for a gene are expressed when present (cattle…red, white, roan coat)

Solving Genetics Problems

There is no dominant or recessive, the heterozygous condition results in a "blending" of the two traits. Example: Snapdragons can be red, white, or pink (heterozygous)

Incomplete dominance - neither allele is dominant, red x white = pink
Codominance - both are expressed in some way, red x white = white/red spots

Use all capital letters - Red (RR) x White (WW) = Pink (RW)

In this graphic, just the letter R is used, with the heterozygous Rr representing the intermediate trait (pink)

It really doesn't matter what letters you choose, as long as the intermediate trait is always the heterozygote.


Example 2

In a certain fish, blue scales and red scales are codominant. When a fish has the genotype B R, it has a patchwork of blue and red scales. What happens if you breed this fish with a fish that only has Blue Scales.

fishStep 1: Determine the genotypes of the parents

B R = patchwork fish

B B = blue fish

R R = red fish

Step 2: Set up the Square


Step 3: Count the number of each phenotype:

1/2 will by patchwork, 1/2 will be blue


What happens when you cross two patchwork fish?