Expression of two allele independently - Codominance

 

Codominance 

• When two alleles of any gene  are present together then each allele expresses itself independently. This phenomenon is called codominance. In this mechanism, the hybrid resembles with the both parents or recieve characters of both parents.

• As a result, the phenotype of the offspring is a combination of the phenotype of the parents. Thus, the trait is neither dominant nor recessive. Blood group AB of ABO blood group system is a classic example of codominance. 

👌👌Remember - Blood group is also example of multiple allele.

• When a trait is controlled by the more than two allele than called as multiple allele and the phenomenon is called multiple allelism

• A person having  alleles IA  and IB exhibit  AB blood group  because IA and IB are Codominant and both are expressed independent.

• The white-spotted red flower in plants and the black-and-white-coated mammals are other examples of codominance.

• In the ABO blood  group system in humans, blood type AB is an example of codominance. In the AB blood group, both dominant alleles are present and expressed together.

• In Human beings, A gene I has three alleles IA,  IB and IO. IA and IB allele are responsible for production of special kinds of sugar that are present on the plasma membrane of RBC or surface of RBC of blood  and make an important part of antigen.

• IA and IB produce A and B types of sugar. Due to the presence of these A and B sugars, blood group AB is determined. Allele IO does not produce any type of sugar.

• Similarly all Four types of blood group are determined on the basis of these A and B sugar present on the RBC surface.

• An individual recieve only two allele out of three because human are diploid in nature.

• A Person with genotype IAIA has blood group A because both alleles produce only A type sugar in blood.

• If A person has Genotype IAIO, that person also has blood Group A because IA produces blood sugar A and IO does not  produce any kind of sugar.

• A Person with genotype IBIB has blood group B because both alleles produce only B type sugar in blood.

• If A person has Genotype IBIO, that person also has blood Group B because IB produces blood sugar B and IO does not  produce any kind of sugar.

• A person has Genotype IOIO, that person also has blood Group O because IO does not  produce any kind of sugar.

• IAIO and IBIO are heterozygous conditions for blood group A and B whereas IAIA and IBIB are homozygous conditions for blood group A and blood group B but for blood group O only homozygous conditions IOIO are present.

• Black and white colour in  animals, codominance also exist. . For example,  when a cross  is made between a black coloured male dog and a white colored  female dog.

• The  offspring with a black-and-white colour are produced. This means that the colours of both the parents are codominant as both are expressed together in the progeny.

• Another example of Codominance is marked in plants that bear white spotted red flowers. A white-spotted red flower produced 

• when a cross is made between a red flower and a white flower. The alleles for the red and white colour both are expressed as codominant alleles and the phenomenon is called codominance.