Beyond Dominant and Recessive Alleles Incomplete Dominance & Codominance

Incomplete Dominance: when neither allele is completely dominant over the other. · Results in the heterozygous phenotype being a blending of the two homozygous phenotypes. · A capital letter represents one of the incompletely dominant alleles. · The same capital letter prime (P’) represents the other incompletely dominant allele so that the two do not get mixed up.

In snapdragons, red flower color (RR) is incompletely dominant to white (R’R’) flower color. A heterozygous snapdragon is pink (RR’). Cross a pink snapdragon with a pink snapdragon.

RR R'R'

F1 GENERATION RR'

F2 GENERATION

______RR' X ______RR' R R'

Genotypic Ratios: R RR RR' 1 RR: 2 RR': 1 R'R' OR 25% RR, 50% RR', 25% R'R'

Phenotypic Ratios: 1 RED: 2 PINK: 1 WHITE 25% RED, 50%, 25% WHITE R' RR' R'R'

In humans, curly hair (HH) is incompletely dominant to straight hair (H’H’). The heterozygous individual has wavy hair (HH’). Cross a person with curly hair with a person who has wavy hair.

______HH X ______HH'

H H

Genotypic Ratios: 2HH: 2HH'

50% HH, 50% HH' H HH HH

2 CURLY: 2 WAVY Phenotypic Ratios: OR H' HH' HH' 50% CURLY, 50% WAVY

Codominance: when both alleles are dominant. · The heterozygous individual shows both alleles expressed equally. · A capital letter represents one of the codominant alleles. · A different capital letter represents the other codominant allele so that the two do not get mixed up.

Roan cattle are codominant:

In chickens, black feather color (BB) is codominant to white feather color (WW). Both feather colors show up in a checkered pattern in the heterozygous individual (BW). Cross a checkered chicken with a checkered chicken. ______BW X ______BW B W

Genotypic Ratios: 1 BB: 2 BW:1 WW

25% BB: 50% BW:25% WW B BB BW

Phenotypic Ratios: 1BLACK : 2 CHECKERED: 1 WHITE 25% BLACK, 50% CHECKERED: 25% WHITE W BW WW

In horses, gray horses (GG) are codominant to white horses (WW). The heterozygous horses(GW) is an appaloosa horse (a white horse with gray spots on the rump and loins). Cross a white horse with an appaloosa horse.

______WW X ______GW G W

Genotypic Ratios: 2GW: 2 WW OR

50% GW : 50% WW W GW WW

W Phenotypic Ratios: 2 APPALOOSA: 2 WHITE OR 50% APPALOOSA : 50% WHITE GW WW