Incomplete Dominance & Codominance

Complete Dominance Recall that in Mendel’s pea plant crosses, one allele was completely dominant over another, a relationship called complete dominance. Heterozygous and dominant homozygous plants have the same phenotype. Example: PP Pp

Incomplete Dominance Occurs when two or more alleles influence the phenotype, resulting in a phenotype intermediate between the dominant and recessive traits. Example: Four o’clock flowers

Codominance Occurs when both alleles for a gene are expressed in a heterozygous offspring. Neither allele is dominant or recessive, nor do the alleles blend in the phenotype. Example: Horse coat color R = red; W = white Red = RR Roan = RW White = WW Roan = Coat consists of both white and red hairs.

Ameoba Sisters Video

Example Snapdragons are incompletely dominant for color; they have phenotypes red, pink, or white. The red flowers are homozygous dominant, the white flowers are homozygous recessive, and the pink flowers are heterozygous. Give the genotypes for each of the phenotypes, using the letters “R” and “ r ” for alleles: a. Red snapdragon _______ b. Pink snapdragon _______ c. White snapdragon _______

a. pink x pink _________ Genotypic ratio _________ Phenotypic ratio

Red x White __________ genotypic ratio __________ phenotypic ratio

Blood Type Blood type is controlled by multiple alleles. 3 alleles = A, B, and O The alleles A and B are codominant when expressed together. O is recessive to A and B. Two of these alleles form an individuals genotype (see chart). Blood Type Genotype Type A AA, AO Type B BB, BO Type AB AB Type O OO