rr Topic: Dominance Keyword: HM60422 Topic: Dominance Keyword: HM60422

Example 5: Incomplete Dominance

Recall that in Mendel's pea-plant crosses, one allele was completely dominant over another, a relationship called complete dominance. In complete dominance, heterozygous plants and homozygous dominant plants are indistinguishable in phenotype. For example, both pea plants PP and Pp for flower color have purple flowers.

Sometimes, however, the F1 offspring will have a phenotype in between that of the parents, a relationship called incomplete dominance. Incomplete dominance occurs when the phenotype of a heterozygote is intermediate between the phenotypes determined by the dominant and recessive traits. In four o'clocks, for example, both the allele for red flowers (R) and the allele for white flowers (r) influence the phenotype. Neither allele is completely dominant over the other allele. When four o'clocks self-pollinate, red-flowering plants produce only red-flowering offspring and white-flowering plants produce only white-flowering offspring. However, when red four o'clocks are crossed with white four o'clocks, all of the F1 offspring have pink flowers. One hundred percent of the offspring of this cross have the Rr genotype, which results in a pink phenotype.

What would be the result of crossing two pink-flowering (Rr) four o'clocks? As the Punnett square in Figure 9-11 shows, the probable genotypic ratio is 1 RR : 2 Rr : 1 rr. Given that neither the allele for red flowers (R) nor the allele for white flowers (r) is completely dominant, the probable phenotypic ratio is 1 red : 2 pink : 1 white.

Example 6: Codominance

Codominance occurs when both alleles for a gene are expressed in a heterozygous offspring. In codominance, neither allele is dominant or recessive, nor do the alleles blend in the phenotype.

For example, the three human MN blood types, M, N, and MN, are determned by two alleles, LM, and LN. The letters M and N refer to two molecules on the surface of the red blood cell. The genotype of a person with blood type MN is LMLN, and neither allele is dominant over the other. Type MN blood cells cary both M- and N-types of molecules on their surface.

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