12.9: Incomplete Dominance

Gregor Mendel's work (1822 - 1884) was primarily focused on pea plants. Through his initial experiments, he determined that every gene in a diploid cell has two variants called alleles inherited from each parent. He suggested that amongst these two alleles, one allele is dominant in character and the other recessive. The combination of alleles determines the phenotype of a gene in an organism.

According to Mendel, organisms with both copies or a single copy of the dominant allele display a dominant phenotype. If the organism has both copies of the recessive allele, it will display a recessive phenotype.

In contrast to Mendel's observations, incomplete dominance or partial dominance can be observed in some genes. It is a phenomenon where both alleles of a gene are partially expressed in a heterozygous organism. Here, the dominant allele is not completely expressed, while the recessive allele influences the trait. This results in a phenotype that is an intermix. For example, in a breeding experiment where one parent is homozygous with a long stem and the other parent is homozygous with a short stem, the offspring of the F1 generation have an intermediate stem length. This is an example of non-mendelian inheritance.

Tay Sachs disease is an example of incomplete dominance in humans. Homozygotes with normal alleles (TT) produce an enzyme called beta-hexosaminidase A that is responsible for breaking down lipids byproducts. Homozygotes with recessive alleles (tt) fail to produce the enzyme. This leads to the accumulation of lipid byproducts in the developing brain of fetuses and young children, resulting in their early death. However, heterozygotes (Tt) for the gene produce half the amount of functional enzymes. Here, both the alleles are expressed, but only one of the alleles produces functional enzymes.

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.