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In some cases, certain phenotype of the offspring shows the resemblance of phenotypes of both parents. This occurs when in heterozygotes, both the alleles of a gene pair are fully expressed. Such a form of inheritance is known as codominance.
- Individuals with type AB blood: Alleles IA and IB are codominant and expressed together, hence individuals inheriting both these alleles will have a type AB blood
- White-spotted red flower in plants
- Black-and-white-coated mammals.
The term ‘codominance’ was coined by the combination of two words: ‘Co-’ which means together and ‘dominance’
Alleles and Genes
Alleles are forms of a given gene variant. Alleles come in pairs. Each pair has its own locus in the chromosome and is responsible for the expression of same trait. The dominant allele is expressed while it masks the expression of recessive allele.
Let’s understand more about codominance by overviewing the basic concept. A gene is the fundamental unit of heredity. Each gene specifies a protein structure or RNA molecule structure. Genes consist of a set of nucleotides that play role in the expression of physical or any other heritable characteristics of an organism.
As mentioned earlier, a variant of a gene is known as an allele. Alleles come in pairs. In a gene, the pair of alleles occupy the same locus in the chromosome and both pairs express the same trait. The allele that is expressed is known as dominant and the allele masked by the dominant one is known as recessive.
Punnet Square helps us to determine possible genotypes of the offspring. It also helps to identify if a trait is codominant. It consists of a diagram with grids and letters which represent alleles in an offspring. A dominant trait is always denoted by an uppercase letter (e.g., A) and a recessive trait by lowercase (e.g., a)
Homozygosity and Heterozygosity
The phenotype of an organism which includes both physical or biochemical characteristics is determined by its genotype. If alleles on the locus are identical, such a condition is known as homozygosity. In contrast, if alleles on the same locus are different, it is known as heterozygosity.
Let’s consider an example of physical characteristics, which is represented by a genotype having alleles A and a. If the individual AA genotype, the condition is homozygous dominant whereas if the genotype is Aa, it is heterozygous dominant. The aa genotype is homozygous recessive.
Dominance in general is a state of being “dominant” or “supreme”. In genetics, a gene or allele that is expressed over the other genes or alleles is known to be dominant. The three major forms of dominance are:
- Complete dominance
- Incomplete dominance
The trait of a dominant allele is expressed when an individual inherits two dominant alleles (e.g., AA) or at least one dominant allele (e.g., Aa). In contrast, if the individual inherits only a recessive allele (e.g., aa), the trait of a recessive allele is expressed. This pattern of inheritance is known as complete dominance and this pattern conforms to the Mendelian inheritance.
The heterozygous organism (Aa) will have a trait similar to the homozygous dominant (AA) as the dominant allele masks the recessive allele. Certain inheritance patterns do not conform to Mendelian inheritance, such patterns of inheritance are known as non-mendelian inheritance.
In some cases, the phenotype of offspring is different from that of their parents. Such a pattern of inheritance is known as non-mendelian inheritance. Examples of such inheritance:
• Codominance: In a heterozygote, if both the alleles of a gene are fully expressed, the resulting phenotype is neither dominant nor recessive. Such a pattern of inheritance is known as codominance.
• Incomplete Dominance: In this form of inheritance, the phenotype of offspring is intermediate to the phenotype of the parents because the dominant allele shows partial expression. The resulting phenotype of the offspring resembles both parents partially. E.g., If a male white flower parent is crossed with a female red flower, the offspring is a plant that bears pink flowers. Thus, in codominance, both alleles are fully expressed while in incomplete dominance, alleles are partially expressed.
Let us understand more about codominance with the help of the following three examples.
i. Type AB blood
Type AB blood of the ABO blood type system in humans is an example of codominance. In this blood type, both the dominant alleles are expressed together. The presence of genotype IAIB results in the production of H antigens with both N-Acetylgalactosamine and galactose.
Alleles IAIB and i control the addition of carbohydrates on H-antigen which is present on the surface of RBC. IA and IB are dominant alleles which on expression produce enzymes that modify the H antigen by adding carbohydrate molecules on it. IA particularly adds N-Acetylgalactosamine’ whereas IB adds galactose on the H antigen.
If an individual has ii alleles, there would be no modifications on the H antigen. Individuals with a genotype of IAIA or IAi have type A blood i.e., they have H antigens with N-Acetylgalactosamine on the red blood cells. Individuals with a genotype of IBIB or IBi have H antigens with galactose – a trait indicating type B blood.
Individuals with a genotype IAIB have type AB blood i.e., they have H antigens with both N-Acetylgalactosamine and galactose. Individual with a genotype of ii means lacking any such modifications on the H antigen – a trait indicating type O blood.
ii. Black and White Coat
In other animals, a mix of coat colors in a progeny of parents with different coat colors is an example of codominance. E.g., A black-and-white coat progeny is produced when a black-furred male dog is crossed with a white-furred female dog. This happens because the alleles responsible for the coat color of both the parents are codominant and hence expressed together in the progeny.
iii. White-spotted Red Flower
If a red flower is crossed with a white flower and the resulting plant bears white-spotted red flowers, it means that the alleles of red and white color are codominant.
Codominance vs Polygenic Inheritance
In codominance, different alleles of a single gene contribute towards a resulting trait. While in polygenic inheritance, multiple genes contribute to producing cumulative effects. Examples of polygenic traits are weight, height, eye color, etc.
- Defining codominance in plant communities. New Phytol . 2021 Jun;230(5):1716-1730.
- Codominance of Lactobacillus plantarum and obligate heterofermentative lactic acid bacteria during sourdough fermentation. Food Microbiol . 2015 Oct;51:57-68.
- Codominance associated with overexpression of certain XPD mutations. Mutat Res . 2001 Mar 7;485(2):153-68.
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