Mendel's traits showed discontinuous variation because they belonged to one or more clear classes. Many other traits show continuous variation because a number of genes affect a single trait, and because environmental factors can influence gene expression. The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait.

Dominance Relations

Incomplete Dominance: is a condition in which the dominant allele cannot completely mask the expression of another allele. For example, red-flowered snapdragons crossed with white ones yield pink in the first generation. The heterozygote phenotype is somewhere between that of two homozyotes. Crossing two heterozygotes results in a 1:2:1 ratio of white:pink:red in the F2. The genotype can be determined by the phenotype, and the intermediate phenotype may be the result of enzyme insufficiency. What does a cross of a heterozygote to a recessive homozygote yield?

This animation (Audio - Important) describes incomplete dominance.

Codominance: is a condition in which both alleles are expressed in heterozygotes.

The gene that controls ABO blood type codes for an enzyme that dictates the structure of a glycolipid on blood cells. Two alleles (IA and IB) are codominant when paired. A third allele (i) is recessive to the others.

IA and IB are each dominant to i, but are codominant to each other. Therefore, some persons can express both genes and have AB blood.

In a blood transfusion, a recipient's immune system will attack blood cells that have an unfamiliar glycolipid on the surface. Type O is a universal donor because it has neither type A nor type B glycolipid.

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