Incomplete Dominance In Biology

Incomplete dominance is a type of inheritance, specifically a type of intermediate inheritance where one allele for a specific trait isn’t entirely expressed (entirely dominant) over its paired allele.

The result is a phenotype (expression) where the expressed physical trait is a combination of both of the phenotypes that belong to the alleles. One allele doesn’t mask or dominate the other alleles in this instance.

Alleles And Mendelian Genetics

Photo: By Pbroks13 – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=5013770

You may have seen a model of inheritance before: the Punnett square. This shows genetic inheritance as a simple model with only two different versions of alleles: dominant and recessive. In this simple relationship, dominant alleles always override the recessive alleles to be expressed in the organism’s appearance or phenotype. This theory of dominant and recessive traits was created by Gregor Mendel and was important because it contradicted popular ideas at the time that the traits of the parents were simply permanently blended within their offspring. However, while this was a good illustration of what Mendel believed at the time, modern biologists have discovered that inheritance isn’t as simple as this model would suggest.

Allele pairs can actually have various different dominance relationships and it is possible that one allele out of the pair may not prevent the other allele from being seen or expressed, at least not completely. Furthermore, there are sometimes many different alleles for a given gene within a specific population.

Mendel conducted a series of experiments with plants and pea plants specifically. When Mendel noticed that the pea plants in his care had either purple or white flowers, he began to wonder why this was the case, and why none of them had a mixed lavender color. Through experimentation, Mendel came to believe that there were two different types of alleles: dominant and recessive.

During the early 1900s, a German botanist by the name of Carl Correns conducted similar experiments to Mendel, although focusing on four o’clock plants. Correns used Mendel’s work as a jumping off point, and it was Correns who is usually credited with the discovery of incomplete dominance. While Correns’ worked with the plants in his care he found that some of them displayed blended colors in their flower petals. Correns deduced that it was likely that every genotype had its own phenotype, and that there was likely a 1:2:1 genotype ratio for many of them. This realization meant that heterozygotic organisms could display both alleles rather than just a single dominant allele.

The set of alleles an organism is referred to as it’s genotype. The fact that the relationship between alleles is not merely dominant or recessive in nature doesn’t mean that in circumstances of incomplete dominance or multiple alleles the alleles don’t determine the phenotype of the organism. The alleles still have an effect on the phenotype, but in these cases, the various alleles can interact with one another in unique ways to create specific phenotypes. In certain cases, the phenotype of a heterozygous organism can be a blended version of its parent’s phenotypes.

Incomplete Dominance And Codominance

An excellent example of the blending of phenotypes is the species of snapdragon called Antirrhinum majus, which will produce pink flowers if homozygous white flowers and homozygous red flowers combine their DNA. This is an example of incomplete dominance. Another example of incomplete dominance in the Andalusian chicken, which is native to Spain and exhibits incomplete dominance in the coloration of its feathers. If a black female Andalusian chicken and a white male chicken breed they will frequently produce offspring who have feathers with blue tinges. This reflects how the mixed alleles dilute the pigment melanin and cause the feathers to be lighter in coloration.

Beyond incomplete dominance, a phenotypic phenomenon called co-dominance can occur. In this case, both alleles are simultaneously expressed win the heterozygotic organism. There are actually groups of people who have a blood type known as MN, and what determines this blood type is the alleles of certain genes. A person with an L^m allele shows an M marker on the surface of their red blood cells, while people with L^n markers show a different N red blood cell marker. While homozygous people only have one of the two markers on their red blood cells, heterozygous people display two of them, a perfect example of co-dominance where both phenotypes are displayed.

Many Alleles

Photo: “Characteristics and traits: Figure 5,” by OpenStax College, Biology (CC BY 3.0).

While Mendel’s work may have suggested that there were only two alleles present for every gene, that’s actually rarely the case in nature. While diploid organisms exist, multiple alleles often exist at the level of the population and different individuals within said population can have different allele pairs. As an example, rabbits frequently have four common alleles of the gene that defines the color of their coat. Not only could a rabbit have a CC pair (which would give them black or brown fur) or a cc pair (which would give them white fur), they could also have a c1c1 or a c2 c2 pair, which would provide them with a multicolored coat and grayish fur respectively.

Polygenic traits are traits like skin color, weight, height and eye colors. Furthermore, polygenic traits are determined by multiple genes and by the interaction of several different alleles with one another. The genes that are responsible for defining these traits have equal influence over the phenotype and different chromosomes contain the alleles for these genes. The effects of alleles that influence polygenic traits are additive in nature, meaning that they each contribute a varying amount of the phenotypic expression. Because of this, it is possible for individuals to have varying degrees of recessive phenotypes, intermediate phenotypes, and dominant phenotypes.

You can think about the way phenotypes are expressed in polygenic circumstances like this:

Individuals who inherited more dominant alleles show a greater expression of that dominant phenotype. Individuals who have inherited more recessive alleles naturally display more of that recessive phenotype. Finally, those who inherited combinations of the recessive and dominant alleles display a phenotype that reflects the contributions from their various dominant and recessive alleles.