Austrian monk, Gregor John Mendel laid the foundation for genetic studies as until the 1800s, genetics was not introduced as a scientific discipline. He conducted his study relying on the famous pea plants, where he discovered complete dominance.
In complete dominance, a heterozygous individual with a dominant allele and a recessive allele for a particular gene shows the phenotypic expression of the dominant allele. In contrast, the expression of the recessive one is masked. A cross made between the pea plants’ traits: round and wrinkled peas, resulted in pea plants with round peas that represented round as a dominant allele and wrinkled as a recessive allele. He described the genotypic ratio to be 1:2:1 and the phenotypic ratio as 3:1.
Until then, scientists believed that a child’s physical traits would always be a combination of his or her parents’ traits. Thus, the phenotype derived from the gene of an organism is affected by dominance; however, the way by which these genes are inherited is not affected.
German botanist Carl Correns (1864-1933) proposed the term “Incomplete Dominance”.
In the early 1900s, he researched a different plant than Mendel’s famous pea plants i.e.“Mirabilis jalapa”, also known as the four o’clock flowers, which contributed to the discovery of incomplete dominance. A cross made between plants with red flowers and the ones with white flowers produced pink flowered progenies. This intermediate heterozygote with pink color flowers was unique and intermediate between that of the parents’ homozygous alleles. Consequently, each genotype had its genotype, and a 1:2:1 genotypic ratio prevailed.
What is Incomplete dominance?
Incomplete dominance, also called partial dominance, semi-dominance, or intermediate inheritance, involves the formation of third phenotypic traits due to the combination of parents’ alleles.
- The incomplete dominance concerns the production of heterozygotes that possess intermediate traits between the two homozygous traits. These heterozygous organisms have phenotypes that are a blend of the phenotypes of their homozygous traits.
- The trait developed is neither dominant nor recessive. Therefore, none of the alleles from the paired ones are expressed over the other for a specific trait.
- The dilution of the dominant allele with respect to the recessive allele is also termed incomplete dominance. There is a reduced ratio of dominant alleles.
- The variation of an organism’s traits or properties is largely influenced by incomplete dominance.
Mechanism of incomplete dominance
The geneticists employ Punnett square to comprehend the mechanism of incomplete dominance. This facilitates anticipation of the genotype of the possible progenies.
Two red alleles that are dominant in the homozygous red flower are denoted by RR. Likewise, rr stands for the homozygous white flower. The above figure illustrates a test cross performed between red and white flower-bearing plants that produce pink flowers on the progeny.
The Punnet square demonstrates that heterozygous offspring with the intermediate characteristic of pink hue is produced in the first filial generation or F1 such that no allele predominates over the others. Their phenotype is intermediate between the two alleles instead of being manifested in a fashion that would obscure the potential effects of the other allele. All of their heterozygous Rr progeny have pink blooms.
For F2 generation, when these heterozygotes are cross-pollinated to observe their corresponding traits, their offspring will be RR, Rr, and rr in the ratio 1:2:1, i.e., some inherit two R alleles in some instances, two r alleles in others, and both R and r in certain cases. This ratio is equivalent to the Mendelian genotypic ratio.
This clarifies that partial dominance does not necessarily mean absolute blending since half of the F2 progenies still demonstrated the parental homozygous alleles, although all of the offspring in the F1 generation were determined to be heterozygous with the third phenotype.
Examples of Incomplete Dominance
Incomplete dominance occurs not only in plants but in animals and humans. Below, several instances are given to have a deeper look at the incidence of this genetic event.
Incomplete dominance in plants
- Incomplete dominance was recorded for the first time in Carnation plants. German scientist Kolreuter crossed true breeding red and white carnation flowers that resulted in offspring with pink phenotypic flowers.
- Red and white flowers of four o’clock plants breed to produce pink flowered progenies where neither allele was fully dominant.
- Similarly, pink-colored snapdragon flowers produced due to cross-pollination between red and white snapdragons exemplify incomplete dominance.
- The light violet color of eggplants is another example of incomplete dominance, which results when deep purple eggplants are combined with white eggplants.
Incomplete dominance in animals
Some of the examples of incomplete dominance in animals are:
- An Andalusian chicken demonstrates incomplete dominance where a white feathered male and a black feathered female chicken breed to produce blue and tinged feathered offspring. This is due to the dilution of genes that reduce the intensity of the effect of melanin, thereby lightening the color of feathers in offspring.
- Breeding of long and short-furred rabbits produces rabbits with varying lengths of fur, usually medium-length furs.
- When a long-tailed dog is crossed with a short-tailed dog, a medium-sized tail is produced in offspring. This demonstrates incomplete dominance.
- Likewise, breeding a spotted animal with a non-spotted animal bears offspring with few spots. This is visible in some animals’ bodies, like dogs, cats, and horses.
Incomplete dominance in humans
This event is particularly rare in humans. However, a few examples are listed below:
- Crossing of parents with straight and curly hair produces children with semi-curly or wavy hair.
- Incomplete dominance is also visible in human height patterns. In contrast to being comparable to either of the parents, children of parents with varied heights tend to be between the parents’ height ranges.
- An illustration of partial dominance in humans is Tay Sachs disease which is an autosomal recessive neurological condition. An enzymatic imbalance could be attributed to incompletely dominant genes in the parents of Tay Sachs’ carrier, causing them to manufacture only half of the required enzyme- enough for a normal life.
- Familial hypercholesterolemia (FH) is a condition that exemplifies incomplete dominance. While liver cells produced by one allele lack cholesterol receptors, those produced by another do so normally. Due to incomplete dominance, cells are produced that lack sufficient receptors to remove harmful cholesterol from the bloodstream completely.