Incomplete Dominance & Codominance

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

Multiple alleles
Polygenic traits
Incomplete dominance
Codominance

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)
The old way still works, though.