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Incomplete Dominance and Codominance • In Mendel’s work, alleles were either dominant or recessive. • If yellow (Y) is dominant to green (y), then: – YY will be yellow – Yy will be yellow – yy will be green • Notice that we only got 2 different phenotypes • However, sometimes we get 3 different phenotypes… Incomplete Dominance • In the case of incomplete dominance, two alleles are neither dominant or recessive • They blend to produce a new phenotype. • For example, when a red snapdragon is crossed with a white snapdragon, all of the F1 offspring are an intermediate color – pink. Example • A certain type of flower will produce green petals when a blue flower is crossed with a yellow flower. Cross two green flowers. BB = blue YY = yellow BY = green BY x BY Genotype: B Y B Y BB BY BY YY 25% = BB 50% = BY 25% = YY Phenotype: 25% = blue 50% = green 25% = yellow Codominance • Codominance occurs when both alleles are equally dominant and both are expressed in a new phenotype. • If a red cattle is crossed with a white one, the offspring are roan. • A roan coat is a mixture of red and white hairs intermingled. • Essentially both alleles are dominant. Multiple Alleles • For each of the traits that Mendel studied, there were only 2 alleles (ex. round/wrinkled, tall/short, etc.) • It is possible to have more than 2 alleles for a trait. • Human blood type is an example of a trait that has multiple alleles (A, B, O) Blood Types • Type O is recessive • Types A and B are dominant to O, but are codominant to each other Blood Type Chart Blood Type Genotype (phenotype) O oo A AA, Ao B BB, Bo AB AB Example • A woman whose blood type is pure A marries a man whose blood type is O. What are the possible blood types of their children? AA x oo Genotype: o o A Ao Ao A Ao Ao 100% = Ao Phenotype: 100% = type A