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Incomplete Dominance

Incomplete dominance occurs when one allele is unable to express its full phenotype in a heterozygous individual. This often causes the heterozygote to have a phenotype that is intermediate to both homozygotes.

In 1760 a German scientist named Josef Kolreuter discovered the example of incomplete dominance that is shown in this animation. He reported on experiments in which he crossed homozygous red carnations (RR) with homozygous white carnations (rr). Therefore, all of the F1 offspring from this cross were heterozygotes (Rr), and should have shown the phenotype of the dominant allele. Surprisingly, they were all pink.

At first this intermediate phenotype was thought to result from a blending of the parents' phenotypes, which would have been strong evidence against Mendel's Law of Segregation. However, when Kolreuter crossed two of the pink (Rr) carnations,1/4 of the F2 generation was red (RR), 1/2 pink (Rr), and the remaining 1/4 was white (rr). As you recall from the section on segregation, this is exactly the genotypic ratio that is expected from a monohybrid (Rr x Rr) cross. That meant that segregation was working just fine.

Thus, in incomplete dominance, it is only the observed phenotypic ratios that depart from those expected in normal Mendelian inheritance because every genotype has its own phenotype.