Incomplete Dominance

Mendel performed thousands of cross-breeding experiments. In one of his most famous experiments, the monohybrid cross, he described the two versions, or alleles, of a single gene. One of these alleles was dominant, while the other was recessive.

In this example, with purple and white pea flowers, only the dominant purple allele appears in the F1 generation.

However, this pattern doesn’t apply everywhere. Sometimes, the F1 heterozygous phenotype is in-between that of the parents.

Consider two snapdragon plants, where one plant is homozygous with red flowers and the other is homozygous with white flowers.

Here, the gene for color is denoted as capital C for color and its allele is placed above the line of text on the gene.

When these two plants are bred, the offspring in the F1 generation are heterozygous with pink flowers.

This type of inheritance, where the offspring’s phenotype is a blend of both parents, is called incomplete dominance.

Here, the allele for the red-colored flower is not completely dominant over the allele for the white-colored flower.

Since neither allele is dominant or recessive, the phenotypic ratio is identical to the genotypic ratio, that is one to two to one.

Hence, a hypothetical F2 generation of four individuals would have one plant with red flowers, two plants with pink flowers, and one plant with white flowers.