Already we have considered genes having only two distinct alleles. If mutation occurs in the  same gene but in different directions in different individuals . The population as a whole will has many different alleles of that gene. Each allele may produce a different phenotype & various combinations of alleles produced several genotypes & phenotype in the population.

Characteristics of Multiple Allele :- 

Defination :- More than two alternative forms of a gene in a population occupying the same locous on a chromosome or its homologue are known as Multiple alleles.

Example :- A well known example of trait determine by multiple alleles in the blood group in man. There are 4 blood groups in human population i.e A,B,AB & O  . This later refer to a glycoprotein substance called an Antigen present on the substance of (RBC) red blood corpuscle . People with blood group A produced A antigen thosw with blood group B form B antigen , those with blood AB produced both antigen & those with blood group no antigen.

The 4 phenotypes A,B,AB & O  are produced by 3 different  alleles. Ia , IB, & Io of the immunogen gene. The allele Ia&iB each of which attaches a different antigen on the surface of RBC . The allele Ia & Ib which  had A&B antigens on the the outer surface of RBC respectively. The alleles Ia & Ib are equally dominant & donot interface with the expression of each other.  The alleles ‘I’ doesnot code for an enzyme and doesnot produced any antigen and is recessive to the other two alleles. Any person carries only two this three alleles.

Any person carries only two this three alleles one from each parents . The blood group A is produced by the genotype Ia ia & iaio the group B by the genotype Ia Ib AND Ib io .
The group AB by the genotype Ia Ib and the group o by the genotype io io. Thus three alleles of gene produced 6 genotype and 4 phenotype.

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