Codominance & Incomplete Dominance

Dominate allele: 

• denoted by a capital letter
• phenotype is observed when homozygotes and heterozygous
• always expressed when present

Recessive allele:

• Denoted by lower case letter
• Phenotype is observed only with homozygous

Codominant:

• bot alleles are fully expressed with heterozygous

Example:

Phenotype-eye color

• blue-dominant (A)
• brown-recessive(a)
• AA – homozygous dominant -> blue
• aA – heterozygous -> blue eyes
• aa – homozygous recessive -> Brown

Example 2:

In chickens, black and white feathers are codominant. Heterozygous chickens have a checkered pattern. A checkered hen is crossed with a checkered rooster. What are the expected ratios of each genotype and phenotype?

1:4 expected to be black
1:2 expected to be checkered
1:4 expected to be white

Incomplete dominance: heterozygous phenotype is between the homozygous phenotypes

Example: Pink flowers when crossing red and white flowers.