Beyond Dominant and Recessive Alleles

• Incomplete Dominance: when neither allele is completely dominant over the other. 

• ·    Results in the heterozygous phenotype being a blending of the two homozygous phenotypes.

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

• __________   X   _________•  • Genotypic Ratios: •  •  Phenotypic Ratios: 

  

 

  

 

• In humans, curly hair (CC) is incompletely dominant to straight hair (SS).  The heterozygous individual has wavy hair (CS).  Cross a person with curly hair with a person who has wavy hair.

• __________    X   _________

• __________    X   _______

  

 

  

  

Genotypic Ratios:  

 Phenotypic Ratios:

• 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.

• 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.

• __________      X      ______

  

 

  

  

Genotypic Ratios:  

 Phenotypic Ratios:

• 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.

• ________      X      ________

  

 

  

 

Genotypic Ratios:   Phenotypic Ratios: