NOTESNOTES - - Simple Simple Dominance, Incomplete Dominance, Incomplete

Dominance, and Dominance, and CodominanceCodominance

Some genes have alleles that are Some genes have alleles that are dominant and recessive, but some do dominant and recessive, but some do not. Genes with alleles that are not not. Genes with alleles that are not dominant or recessive create more than dominant or recessive create more than two phenotypes.two phenotypes.

Simple DominanceSimple Dominance – One allele of the – One allele of the gene is dominant, one is recessivegene is dominant, one is recessive

With this type of inheritance, there can With this type of inheritance, there can only be 2 possible phenotypesonly be 2 possible phenotypes

Ex. Ex. Eye color (Eye color (BB – non-blue eyes, – non-blue eyes, bb – blue eyes) – blue eyes)

BBBB = non-blue eyes = non-blue eyes BbBb = non-blue eyes = non-blue eyesbbbb = blue eyes = blue eyes

Incomplete DominanceIncomplete Dominance – Neither allele is – Neither allele is dominant so heterozygous organisms have a dominant so heterozygous organisms have a mixed phenotypemixed phenotype With this type of inheritance, there are 3 With this type of inheritance, there are 3

possible phenotypespossible phenotypes Since neither allele is dominant, lowercase Since neither allele is dominant, lowercase

letters are used to represent the genesletters are used to represent the genes Ex. Flower color in snapdragons (Ex. Flower color in snapdragons (rr – red flowers, – red flowers,

ww – white flowers) – white flowers)

rrrr = red flowers = red flowers wwww = white flowers = white flowers rwrw = pink flowers = pink flowers

CodominanceCodominance – – Both alleles are dominant, Both alleles are dominant, heterozygous organisms show both traits in heterozygous organisms show both traits in their phenotypetheir phenotype With this type of inheritance, there are 3 With this type of inheritance, there are 3

possible phenotypespossible phenotypes Since both alleles are dominant, uppercase Since both alleles are dominant, uppercase

letters are used to represent the genesletters are used to represent the genes Ex. Feather color in Erminette chickens (Ex. Feather color in Erminette chickens (B B – –

black feathers, black feathers, WW – white feathers) – white feathers)

BBBB = black feathers = black feathers WW WW = white = white feathers feathers BWBW = black & white feathers = black & white feathers