Dominance: what it is and what it is not

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Dominance is the relationship between phenotype and genotype

Complete dominance: the phenotypes of AA and Aa are the same

e.g., Mendel's YY and Yy had the same phenotype, yellow

Incomplete dominance: the phenotype of Aa is intermediate between the phenotypes of AA and aa

e.g., In snapdragons, the phenotype of Rr is pink, which is intermediate between RR (red) and rr (white)

Incompletely dominant genes may also be additive, if there is a proportional correlation between the intensity of the phenotype and the number of dominant alleles (in the above example, 2 dominant alleles is "twice as red" as 1 dominant allele)

Co-dominance: the phenotype of Aa is a combination of the phenotypes of AA and aa

e.g., In humans, individuals with the genotype I^AI^B express both the A and the B antigen on their red blood cells

Dominance is not directly correlated with a higher fitness, as mistakenly assumed by people who challenged Punnett with the famous question: "Don't dominant alleles take over the population over time?"  Genotypes with higher fitnesses (w) increase in frequency over time as a result of selection.  These genotypes need not be associated with the dominant allele.

However, given that the fitness  of a genotype depends on the effect of selection on the phenotype, the relationship between phenotype and genotype (dominance) will affect how selection acts on genotypes changes through time.  Accordingly, fitness and dominance are not completely independent either.  As we shall see in the section on selection, the following are examples of such relationships:

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