1. Incomplete Dominance & Codominance

2. 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

3. 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

4. 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.

5. Ameoba Sisters Video

6. 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 _______

7. a. pink x pink
_________ Genotypic ratio
_________ Phenotypic ratio

8. Red x White
__________ genotypic ratio
__________ phenotypic ratio

9. 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