A single allele becomes totally dominant over the other allele in the case of complete dominance. The phenotype of the heterozygote appears similar to the phenotype of the homozygous dominant. On the other hand, complete recessiveness requires two recessive alleles, meaning the recessive allele gets expressed only in the homozygotes. Hence, these two extremities get classified under a range of dominance relationships. Main topics covered under the dominance relationships include complete dominance, incomplete dominance, codominance, and the molecular explanations related to the same.
Complete dominance:
One allele has a powerful effect of masking the activity of the other allele. It is known as complete dominance. One allele dominates the other allele thereby masking its expression. The allele getting masked is known as a recessive allele. Hence, the phenotype of the heterozygote shows similarity with the phenotype of the dominant homozygote. Let us consider the example of complete dominance. Seed shape in peas helps in determining the dominant alleles present in the pea plant. Round peas have R allele, the dominant one. The wrinkled peas have r allele, the recessive one. Hence, the combination of alleles includes RR (having round peas), rr (wrinkled peas), and Rr (Round peas).
Incomplete dominance:
Incomplete dominance, also known as partial dominance, occurs when one allele lacks complete dominance over the other allele. It usually becomes partially dominant. Since there is an incomplete dominance, the heterozygote expresses a phenotype intermediate to that of the homozygotes having either of the alleles. Let us consider an example of incomplete dominance in chickens. The plumage or feather colors of the chickens vary. It is due to incomplete dominance. Consider a cross between the true breeding black strain and a true breeding white strain. The true breeding black strain is a homozygote and consists of CBCB alleles. The true breeding white strain is also a homozygote showing CWCW alleles. A cross between both of them gives rise to an F1 generation of birds having bluish-grey plumage. They are also known as Andalusian blues. They show a genotype of CBCW. The letter C symbolizes the color of the plumage and the letters W and B symbolize white and black colors respectively. Consider another cross between Andalusians. In this case, both the Andalusian blues have CBCW genotype. The F2 generation now shows 1:2:1 phenotypic ratio. The progeny involves one black, one white, and two Andalusian blue fowls.
Let us consider one more example of incomplete dominance. The Palomino horse has a golden yellowish brown body color with a white mane and a tale. Interbreeding of the Palominos gives rise to the progeny with a phenotypic ratio of 1:2:1. The F2 generation consists of two palominos, one cremellos, and one light chestnut.
Image: Incomplete dominance
Codominance:
The phenotype of the heterozygote resembles that of both the homozygotes in codominance. Codominance and Incomplete dominance are related to one another. Codominance is a form of modified dominance relationship. Example of codominance involves ABO blood groups. A glycoprotein known as the H antigen occurs on the surfaces of the blood cells. Three main alleles control the chemical modification of the glycoprotein. Two out of three alleles are codominant. They are known as IA and IB respectively. These alleles are dominant over the recessive “i” allele. Both IA and IB code for different enzymes. The one coded by IA adds an N-acetylgalactosamine to the H antigen. The one encoded by IB adds galactose. The recessive i allele does not modify anything.
In the ABO system of blood groups, there are four main phenotypes such as O, A, B, and AB. Individuals with A blood group express antigen A. Individuals with B blood groups express B antigen. Individuals with AB blood group express both the antigens. The individuals with O blood groups express none of the antigens. Out of the four types, the AB blood group individuals produce A and B antigens. Hence, it is an example of codominance. Another example of codominance involves MN blood group system. Let us consider another example of codominance. Sickle cell anemia is a type of hemoglobinopathy. Three molecular types of hemoglobin include HbA/HbA, HbA/HbS, and HbS/HbS.
Molecular explanation
Both the homozygote phenotype so gets expressed in a heterozygote since it has both the alleles. It is a general explanation of codominance. However, incomplete dominance results in the expression of only one allele out of the two in a heterozygote. Hence, a homozygote consists of two doses of the gene product. Here we talk about haplosufficiency. Heterozygote showing normal dominance requires only half of the homozygous allele for expression of the phenotype. Dominance occurs due to the presence of a non-functional allele. Meaning, one allele loses its functions such as expressing a protein product or skips the process of transcription. It also occurs due to mutation altering the DNA sequence. Hence, the organism having a non-functional allele will show a particular phenotype. Example, albinism is an autosomal recessive disorder occurring due to the inefficient synthesis of the pigment melanin in the skin and the hair. Hence, such individuals show whitish-pink skin coloration. Let us consider the interaction between the alleles that make an allele recessive. The single allele produces a product or a phenotype identical to that of the homozygote (haplosufficiency). Hence, the functional allele becomes dominant to the non-functional one. In the case of an albino gene locus, the heterozygous individuals produce sufficient melanin in the skin. Hence, they exhibit normal pigmentation in the skin.
Consider another interaction involving haploinsufficiency. The phenotype expressed by the functional allele involves less severity when compared with that of a non-functional homozygote. Rarely may it also produce an insufficient gene product as compared to that of a non-functional homozygote. Sometimes, the non-functional allele may also produce defective protein products interfering with the normal protein functioning.
Dominant negative mutations mostly arise in the somatic cells. These mutations are harmful to the body. They provide scope for the mutant cells to proliferate and expand. Most of the dominant negative mutations a cancer cell or a mutant cell resistant to the natural cell death process known as apoptosis. Hence, the damaged DNA in these cells does not get repaired. Example, dominant negative mutations occur in the tumor suppressor gene such as p53. The p53 mutations occur in different types of cancer cells such as cells present in the breast, prostate, and the brain.
References:
[1] Principles Of Genetics 7/E, By Tamarin
[2] Essential Genetics: A Genomics Perspective, Hartl, Elizabeth W. Jones
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