Difference between Incomplete Dominance A Co-Dominance

What is the meaning of dominance in genetics?

The term dominance refers to an event in which one variant of a gene is superior and overrides a different variant of the same gene; such a variant is called a dominant factor. The other factor is called a recessive factor in a gene. The presence of two different variants in the chromosome is caused by mutations that can be inherited or new.

How and when was the concept of dominance discovered?

The concept of dominance was discovered and introduced to the scientific world by Gregor Johann Mendel in 1860. Still, it came into notice only in the early 1900s. He was known as the father of genetics for his groundbreaking findings in genetics.

While experimenting with peas, he observed two discrete phenotypes: round versus wrinkled seeds, yellow versus green seeds, red versus white flowers, or tall versus short plants in the pea plants. He found that if the plants with the same characteristics are self-pollinated repeatedly, they will produce the same phenotype. Still, if a plant with different characteristics is crossed, they will show different results in the F2 generation of the phenotype.  

For example:

The ratio in the F2 generation always comes as 3:1 in a monohybrid cross where tall plants are represented by (T) and short is shown by (t). when these two types of plants are crossed, we get the combination of Tt, which is a tall plant, so we get to know that (T) is the dominant factor in the gene which is suppressing the other factor (t), the recessive factor, now when the F1 generation is self-crossed, we get the variation of TT, Tt, Tt, tt i.e. one short plant and three tall plants.

Law of dominance:

Law of dominance states that:

(i) Characters are controlled by discrete units called factors.

(ii) Factors occur in pairs.

(iii) In a dissimilar pair of factors, one member of the pair dominates (dominant) the other (recessive). 

The law of dominance is used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2. It also explains the proportion of 3:1 obtained at the F2 generation.

Types of Dominance:

There are 3 types of dominance, namely:

1. Complete dominance: In this type of dominance, the dominant factor completely suppresses the recessive factor and is indifferent from the original plant.

An example of dominance is the inheritance of peas’ seed shape (pea shape). Peas may be round (R) or wrinkled (r). In this case, three combinations of alleles (genotypes) are possible: RR and rr are homozygous and Rr is heterozygous. The RR individuals have round peas and the rr individuals have wrinkled peas. In Rr individuals, the R allele masks the presence of the r allele, so these individuals also have round peas. Thus, allele R is dominant to allele r, and allele r is recessive.

2. Incomplete dominance: In this type of dominance, the F2 generation of the plants does not look like their parents or are intermediaries of their parents.

3. Co- dominance: In this type of dominance, both the factors are dominant and visible

For example, ABO blood groups are controlled by gene I. This gene controls our sugar. The gene (I) has three alleles I A, I B and i. The alleles I A and I B produce a slightly different form of sugar, while allele i do not produce any sugar. Each person possesses any two of the three I gene alleles. I A and I B are utterly dominant over i. In other words, when I A and i are present, only I A expresses (because i does not produce any sugar). When I B and i are present, I B expresses. But when I A and I B are present, they both express their own types of sugars because of Co-dominance.

Difference between Incomplete Dominance and Co-dominance:

Parameters Of Comparison

• Incomplete Dominance

• Co-dominance

Incomplete dominance is seen in heterozygotes, where the dominant trait does not overcome the recessive character, whereas, Co-dominance is seen in heterozygote, where the dominant and recessive traits are expressed in their characteristics.

Phenotype expressed —

Incomplete Dominance: Different looking offspring from their parents

Co-dominance: Identical looking offspring as their parents 

Dominance —

Incomplete Dominance: One of the allele’s traits gets dominant over another

Co-dominance: Both of the alleles don’t act as dominant or recessive over another

Examples —

Incomplete Dominance: Crossing of red flowers

Co-dominance: ABO Blood group

Conclusion:

In this article, we learnt about dominance, where one variant can act as a dominant while the other variant as recessive in the same genes. Also different types of dominance can be seen, how the two variants of same genes can act differently as per the given environment or on the type of situation.

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