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?

B W
B BB BW
W BW WW

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.