Gregor Mendel knew how to keep things simple. In Mendel’s work on pea plants, each gene came in just two different versions, or alleles, and these alleles had a nice, clear-cut dominance relationship (with the dominant allele fully overriding the recessive allele to determine the plant’s appearance).
Today, we know that not all alleles behave quite as straightforwardly as in Mendel’s experiments. For example, in real life:
Allele pairs may have a variety of dominance relationships (that is, one allele of the pair may not completely “hide” the other in the heterozygote).
There are often many different alleles of a gene in a population.
In these cases, an organism’s genotype, or set of alleles, still determines its phenotype, or observable features. However, a variety of alleles may interact with one another in different ways to specify phenotype.
As a side note, we’re probably lucky that Mendel’s pea genes didn’t show these complexities. If they had, it’s possible that Mendel would not have understood his results, and wouldn’t have figured out the core principles of inheritance which are key in helping us understand the special cases!
What is Incomplete Dominance?
Definition: “Incomplete dominance is a form of intermediate inheritance in which one allele for a particular trait is not expressed completely over its paired allele.”
Incomplete dominance is a form of Gene interaction in which both alleles of a gene at a locus are partially expressed, often resulting in an intermediate or different phenotype. It is also known as partial dominance.
For eg., in roses, the allele for red colour is dominant over the allele for white colour. But, the heterozygous flowers with both the alleles are pink in colour.
Discovery Of Incomplete Dominance
Scientists have noted the blending of traits back into ancient times, although until Mendel, no one used the words “incomplete dominance.” In fact, Genetics was not a scientific discipline until the 1800s when Viennese scientist and friar Gregor Mendel (1822–1884) began his studies.
Like many others, Mendel focused on plants and, in particular, the pea plant. He helped define genetic dominance when he noticed that the plants had either purple or white flowers. No peas had lavender colors as one might suspect.
Up to that time, scientists believed that physical traits in a child would always be a blend of the traits of the parents. Mendel proved that in some cases, the offspring can inherit different traits separately. In his pea plants, traits were visible only if an allele was dominant or if both alleles were recessive.
Mendel described a genotype ratio of 1:2:1 and a phenotype ratio of 3:1. Both would be consequential for further research.
While Mendel’s work laid the foundation, it was German botanist Carl Correns (1864–1933) who is credited with the actual discovery of incomplete dominance. In the early 1900s, Correns conducted similar research on four o’clock plants.
In his work, Correns observed a blend of colors in flower petals. This led him to the conclusion that the 1:2:1 genotype ratio prevailed and that each genotype had its own phenotype. In turn, this allowed the heterozygotes to display both alleles rather than a dominant one, as Mendel had found.
Mechanism of Incomplete Dominance
Incomplete dominance occurs because neither of the two alleles is completely dominant over the other. This results in a phenotype that is a combination of both.
Gregor Mendel conducted experiments on pea plants. He studied on seven characters with contrasting traits and all of them showed a similar pattern of inheritance. Based on this, he generalized the law of inheritance.
Later, researchers repeated Mendel’s experiment on other plants. Shockingly, they noted that the F1 Generation showed variation from the usual pattern of inheritance. The monohybrid cross resulted in F1 Progeny which didn’t show any resemblance to either of the parents, but an intermediate progeny.
Let’s understand the incomplete dominance with the example of Hibiscus flower (Hibiscus sp).
Monohybrid cross was done between the red and white coloured flowers of Hibiscus plant. Consider, pure breed of the red flower has RR pair of alleles and that for the white flower is rr.
Firstly, true-breeding red (RR) and white (rr) coloured flowers of Hibiscus were crossed. The F1 generation produced a pink coloured flower with Rr pair of alleles.
Then the F1 progeny was self-pollinated. This resulted in red (RR), pink (Rr) and white (rr) flowers in the ratio of 1:2:1.
Recollect that the genotype ratio of F2 generation in the monohybrid cross by Mendel also gave the same ratio of 1:2:1. However, the phenotype ratio has changed from 3:1 to 1:2:1. The reason for this variation is the incomplete dominance of the allele R over the allele r. This led to the blending of colour in flowers.
Incomplete Dominance and Codominance
The laws of inheritance proposed by Mendel (Mendelian inheritance or Mendelian genetics) defined the dominance factors in inheritance and the effects of alleles on the phenotypes of diploid organisms.
Codominance and incomplete dominance are different types of inheritance (specifically genetic). However, both incomplete dominance and codominance types of dominance were not identified by Mendel.
However, his work leads to their identification. Several botanists worked in the inheritance field and found these respective dominance types. Both incomplete dominance and codominance are often mixed up. Therefore, it is important to see the primary factors that lead to differing from each other.
Incomplete dominance
As mentioned earlier, incomplete dominance is partial dominance, meaning the phenotype is in between the genotype dominant and recessive alleles.
In the above example, the resulting offspring has a pink color trait despite the dominant red color and white color trait due to incomplete dominance.
The dominant allele does not mask the recessive allele resulting in a phenotype different from both alleles, i.e., pink color. The incomplete dominance carries genetic importance because it explains the fact of the intermediate existence of phenotype from two different alleles.
Moreover, Mendel explains the Law of dominance that only one allele is dominant over the other, and that allele can be one from both. The dominating allele will reduce the effect of the recessive allele.
Whereas in incomplete dominance, the two alleles remain within the produced phenotype, but the offspring possess a totally different trait. Mendel did not study incomplete dominance because the pea plant does not show any incomplete dominance (intermediate traits).
However, Mendel’s proposed ratio of 1:2:1 tends to be accurate for incomplete dominance, as seen in the example of the four o’clock flower, where the F1 generation results in red, pink, and white flowers’ genotypic ratio of 1:2:1, respectively.
These results show the Law of inheritance where alleles are inherited from parents to offspring still occurs in the incomplete dominance described by Mendel.
In research on quantitative genetics, the possibility for incomplete dominance requires the resulting phenotype to be partially related to any of the genotypes (homozygotes); otherwise, there will be no dominance.
Codominance
Codominance refers to the dominance in which the two alleles or traits of the genotypes (of both homozygotes) are expressed together in offspring (phenotype).
There is neither a dominant nor recessive allele in cross-breeding. Rather the two alleles remain present and formed as a mixture of both of the alleles (each allele has the tendency to add phenotypic expression during the breeding process).
In some cases, the codominance is also referred to as no dominance due to the appearance of both alleles (of homozygotes) in the offspring (heterozygote). Thus, the phenotype produced is distinctive from the genotypes of the homozygotes.
The upper case letters are used with several superscripts to distinguish the codominant alleles while expressing them in writing. This writing style indicates that each allele can express even in the presence of other alleles (alternative).
The example of codominance can be seen in plants with white color as a recessive allele and red color as the dominant allele producing flowers with pink and white color (spots) after cross-breeding.
Similarly, Mendel also did not consider the codominance factor due to the pea plant’s limited traits. However, further research revealed the codominance in plants and vice versa. The genotypic ratio was the same as Mendel described.
They produced offspring that resulted in the F1 generation including red, spotted (white and pink), and white with the same genotypic ratio.
Codominance can be easily found in plants and animals because of color differentiation, as well as in humans to some extinct, such as blood types.
The incomplete dominance produces offspring with intermediate traits whereas the codominance involves the mixing of allelic expressions.
However, in both types of dominance, the parent alleles remain in the heterozygote. Nonetheless, no allele is dominant over the other.
Incomplete dominance vs. Codominance
| Incomplete dominance | Codominance |
| Incomplete dominance occurs in the heterozygote, in which the dominant allele is incompletely dominant, meaning it does not dominate the recessive allele entirely; rather, an intermediate trait appears in the offspring. | Codominance occurs when the alleles do not indicate any dominant and recessive allele relationship. However, each allele from homozygote is able to add phenotypic expressions in the offspring or simply the “mix” of each allele. |
| The offspring’s phenotype is an intermediate of the parents’ homozygous traits. | The phenotypic expression of homozygous in codominance is independent. |
| The expression of alleles in incomplete dominance is conspicuous, meaning none of the alleles dominates over the other. | The expression of alleles in codominance is uniformly conspicuous, meaning both alleles have an equal chance of expressing their effects. |
| The formed trait (phenotype) is different due to mixing both parents’ phenotypes and genotypes. | The formed trait (phenotype) is not different due to the no mixing of both parents’ phenotypes and genotypes. |
| The offspring do not show the parental phenotype. | The offspring shows both parental phenotypes. |
| The dominant allele does not dominate over the recessive allele. | The offspring phenotype produced possesses the combination of two alleles and, thus, shows two phenotypes together. |
| The dominant allele does not dominate over the recessive allele. | None of the alleles is dominant or recessive, and the dominating relationship does not occur. |
| The quantitative approach can be used for the analysis of incomplete dominance in organisms (including the analysis of both non-dominating alleles). | The quantitative approach can be used for the analysis of codominance in the organism (only including the analysis of gene expressions). |
| Incomplete dominance examples include Pink flowers of four o’clock flowers (Mirabilis jalapa), and physical characteristics in humans, such as hair color, hand size, and height. | Codominance can be seen in humans as well as in animals. The red blood cells/blood type (or groups A, B, and O) in humans and the spots on the feathers or hairs of livestock are examples of codominance. |
Examples of Incomplete Dominance
Examples of incomplete dominance are mentioned below:
In Humans
The child of parents each with curly hair and straight hair will always have wavy hair. Carriers of Tay-Sachs disease exhibit incomplete dominance.
In Other Animals
The Andalusian chicken shows incomplete dominance in its feather colour.
When the rabbits with long and short furs are bred, the offsprings produced will have medium fur length.