Beyond Dominant and Recessive Alleles

The whole story... Not all phenotypes are dictated by dominant and recessive alleles (in fact, very few are). Some alleles are neither dominant nor recessive. Most phenotypes are controlled by the following mechanisms: Incomplete Dominance Codominance Multiple Alleles Polygenic Traits

Incomplete Dominance When one allele is not completely dominant over another and the heterozygous phenotype is an intermediate (blending) between the two homozygous phenotypes. Example: WW = White Flowers RR = Red Flowers RW = Pink Flowers

Codominance Similar to incomplete dominance, except that no new phenotype is created (not a blending of the other phenotypes). Instead, both of the homozygous phenotypes are being expressed simultaneously Example: C B C B = Black feathers C W C W = White feathers C B C W = has some Black and some White feathers

Multiple Alleles Occurs when there is more than 2 possible alleles that can be inherited from the parents. (Each parent only has 2 alleles, but in this case there are several possible types of alleles.)

ABO Blood Groups There are 3 possible alleles for blood type: I A, I B, and i Alleles I A and I B are codominant, while i is recessive.

Polygenic Traits In polygenic traits, more than one gene is used to determine the genotype and phenotype of a particular trait (ex. skin color). This allows for an extremely large variety of phenotypes.