Difference Between Incomplete Dominance and Codominance

In genetics, Dominance is a relationship between alleles of one gene. In order to understand the concept of the dominance of alleles, we need to know more about genes.

So far, we know that genes are a hereditary unit in organisms that exist as a pair of alleles in diploid organisms. These pairs of alleles may or may not be similar. That is, a heterozygous gene has two dissimilar pairs of alleles, while homozygous genes have identical ones.

Heterozygous alleles carry different information on traits. When we say one trait is dominant over the opposite, there are often two reasons:

• either it is non-functional, or

• is less active than the normal allele

Incomplete Dominance and Codominance

Incomplete dominance and codominance are different from one another.

In codominance, both the alleles present on a gene are expressed in the phenotype. A flower showing codominance will have patches of red and white instead of a uniformly pink flower.

In incomplete dominance, the F2 generation from heterozygous plants will have a ratio of 1:2:1 with the phenotypes red, white and spotted flowers.

The humans with AB blood type also show codominance where the alleles for both blood types A and B are expressed.

Introduction of Incomplete Dominance

The process in which two true-breeding parents crossed to produce an intermediate offspring is called incomplete dominance. Incomplete dominance is referred to as partial dominance or intermediate inheritance. In incomplete dominance, the variants, also called alleles, are not expressed as dominant or recessive. Instead, the dominant allele is expressed in a reduced ratio. An allele is a form or set of gene expressions. Generally, an organism consists of two alleles inherited from each parent. The type of allele that masks or suppresses other alleles and becomes prominent in the offspring is called a dominant allele. The type of allele that is suppressed by the dominant allele and does not appear in the offspring is called a recessive allele. Multiple alleles is a type that refers to the various alleles, two or more than two, for the same gene.

Incomplete dominance can also be called semi-dominance and partial dominance. Incomplete dominance is the phenomenon that occurs because neither of the two alleles is completely dominant over the other. And it results in a phenotype that is a combination of both. The generation of a third phenotype, specifically with traits that results from the combination of parent alleles, is called incomplete dominance. The incomplete dominance is also referred to as intermediate inheritance in terms of trait expression, and no other alleles from the paired alleles are expressed over the other for a specific trait. Incomplete dominance is defined as a lack of dominance in either of two different alleles in heterozygotes so that the phenotype is intermediate between that of homozygotes for either of the two alleles. A type of organism that possesses two same alleles for a specific gene and can truly breed for the allele is described as homozygous. And the organism that possesses two different alleles for a specific gene is described as heterozygous. A whole set of genes in an organism that is inherited by the offspring and that determines the offspring’s observable physical features is called the genotype. And on the other hand, Phenotype is determined by the genotype and refers to the organism’s appearance, characteristics, behaviour, and development.

What is Incomplete Dominance?

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.

Mechanism of Incomplete Dominance

Incomplete dominance occurs because neither of the 2 alleles is totally dominant over the opposite. This results in a phenotype that is a combination of both.

Gregor Mendel conducted experiments on pea plants. He studied 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. 

The incomplete dominance on the Snapdragon flower, Monohybrid cross was done between the red and white coloured flowers of the Snapdragon plant. Consider that the pure breed of the red flower has an RR pair of alleles and that for the white flower is rr. At first, true-breeding red (RR) and white (rr) coloured flowers of snapdragon were crossed. Then after that, the F1­ generation produced a pink coloured flower with Rr pair of alleles. And it resulted that the F1 progeny was self-pollinated. This will result in red (RR), pink (Rr) and white (rr) flowers in the ratio of 1:2:1. The genotype ratio of F2 generation in the monohybrid cross by Mendel also gave the same ratio of 1:2:1 and the phenotype ratio has changed from 3:1 to 1:2:1. The reason for this generation is the variation of incomplete dominance of the allele R over the allele r. This will automatically lead to the blending of colour in flowers.

Examples of Incomplete Dominance

Examples of incomplete dominance are mentioned below:

In Humans

The fact that the concept of adaptation of incomplete dominance by humans in genetics to increase better living, the incomplete dominance can be seen in humans genetically. The crossing of two different types of alleles in the genetic process produces the process of human offspring either with different or intermediate forms between the two traits. And it can be said that incomplete dominance is as old as a human life that leads to variation with time. 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

Generally, in some animals or birds, this phenomenon of incomplete dominance is also visible. Various examples of incomplete dominance can be seen in chickens, cats, rabbits, dogs and horses. The other example for incomplete dominance includes the spots on animals bodies and are more visible in cats, dogs, and horses. When they are bred, these animals will produce offspring with varying spots.

In Plants

The Carnation plant, an example of incomplete dominance, has true-breeding white flowers and true-breeding red flowers. A cross between white-flowering and red-flowering carnation plants may result in offspring with a phenotype of pink flowers. In plants, basically, the self-sterility n is an example of multiple alleles that causes the rapid growth of pollen tubes.

Introduction of Codominance

Codominance refers to the phenomenon in which dominance of the two alleles or traits of the genotypes is expressed together in offspring. In Codominance, there is neither a dominant nor recessive allele in cross-breeding. Instead, the two alleles remain present and formed as a mixture of both of the alleles that leads to each allele having the tendency to add phenotypic expression during the breeding process. In many cases, codominance is also defined as no dominance due to the appearance of both alleles in the offspring.

The phenotype produced is different from the genotypes of the homozygotes. Codominance can be easily found in animals and plants because of colour differentiation, as well as in humans to some extinct, for example, blood type. The codominance involves a mixture of allelic expressions. In both types of dominance, Incomplete dominance and Codominance, the parent alleles remain in the heterozygote. No allele is dominant over the other.

Difference Between Incomplete Dominance and Codominance