Co-Dominance

Introduction

In this article, we will introduce the topic of co-dominance. We have introduced the concept of co-dominance from Biology. In co-dominance, both the gene expressed for a particular character in F1 hybrid progeny. There is no blending of characters, whereas both the characters are expressed equally. Let’s start with the types of dominance.

Co-Dominance 

Dominance is shown in three ways: complete dominance, incomplete dominance and co-dominance. Co-dominance refers to the relationship in the two versions of a gene. It is a type of dominance. An allele is a version of a gene. Each individual receives it from their parents. The dominant allele will be if the alleles are different. Recessive is the effect of the other allele. It is masked. When the alleles are expressed in the same phenotype, this situation is called co-dominance. Co-dominant alleles are always shown using different letters in upper case letters in genetic diagrams. If we take an example of hens, the feather colour may be black, white or both (speckled has both the features black and white). We have shown colours by W and B, which are for white and black respectively.

There can be three possible genotypes: BB, WW, and BW.

There also can be three possible phenotypes – BB = Black, WW = White and BW = Speckled.

 Heterozygous is meant by having two different alleles. It refers to the different forms of a particular gene from each  parent in inheritance. In a heterozygous genotype, the dominant allele over-rules the recessive one. Therefore, the dominant trait will be expressed. 

For example, a heterozygous genotype is a pea plant. Pea plants may have flower  homozygous dominant (red, genotype RR). If they have white flowers, then they will be homozygous recessive (white, genotype rr). If pea plants may have  flowers heterozygous (red, genotype Rr ). Dominant trait expressed in heterozygous form. This example is presenting the complete dominant. 

Co-dominance trait with example:-

Co-dominant is formed by two words co and dominant. Co means with (cum) and dominance means dominance.

The co-dominant trait is a trait resulting from an allele and the allele is equally and independently expressed along with each other. Blood type is an example of a co-dominance trait. If a person has a blood type AB, person will have one allele of blood type B and another allele of blood type A.

Genes are the heredity units of a person and often determine the features of a person. Genes are found in chromosomes and are responsible for a person’s appearance. Genes are present in all living creatures and humans, responsible for many characteristics such as eye colour, hair colour etc.

For example – a flower can be pink, red or other colours due to alleles.

Dominant alleles are alleles in which characteristics and presence have an appearance. And recessive alleles do not have the appearance of characteristics. In recessive alleles, characteristics are hidden by the dominant alleles.

When there are no dominant alleles and recessive alleles or both are present in the same ratio/form, it is called co-dominance. Some examples of co-dominance are:-

1) AB blood type – blood type is determined by the ABO gene. And every person has two copies of the ABO gene and it is received from each parent. There can be many combinations of blood types.  Persons have both  A and B proteins. A person will have a blood group AB when he has both the A and B allele.

2) Horse colour – horses have a roan coat colour due to dominance. There are three types of roan in horses: bay, blue and red. All the colours follow similar Co-dominance patterns.

3) Flower colour – the petal colour with red paint and white flecks of  plants(Camellias). It expressed codominance.

4) Sickle–cell anemia – It is a disease that arises when RBCs (red blood cells) become stretched out and thin. Co-dominance occurs when both sickled and regular shapes are mixed and seen in the blood.

Incomplete Dominance and Co-dominance Classification 

Sometimes co-dominance and incomplete dominance have the same meaning or some are confused between their meanings, but there is no need to get confused; both are different. Different phenotypes result from co-dominance, incomplete dominance and complete dominance.

Co-dominance is expressed in different types of an animal by two phenotypes. For example – Holstein cows have white and black spots resulting from the black and white alleles. Each is fully expressed in different types of organisms. While two alleles are mixed or integrated to create a different phenotype in incomplete dominance. For example,  white allele and red alleles of flowers are mixed to produce the pink phenotype. So the pink phenotype is a mixture of both the alleles expressed in every cell. 

If incomplete dominance is expressed in the Holstein cow, all the cows would be gray instead of having black and white spots.

When Gregor Mendel studied polygenic traits and their inheritance, he discovered co-dominance. According to his law of inheritance, an allele of a gene can either be recessive or dominant. Incomplete dominance is discovered by Karl Correns, in which any factor of a gene is not dominant. The blend of dominant and recessive traits are the  phenotype of a heterozygous dominant  Co-dominance is an exception to Mendel’s law  of inheritance

Conclusion

Co-dominance refers to the relationship in the two versions of a gene. It is a type of dominance. An allele is a version of a gene. Each individual receives it from their parents. The co-dominant trait is a trait resulting from the allele and it is independent and equally expressed. There are four examples of co-dominance: AB blood group system, horse colour, flower colour and sickle–cell anemia. And the difference between co-dominance and incomplete dominance is also shown. In incomplete dominance, two or more alleles are mixed to give a different allele type, while two alleles are expressed in Co-dominance.