Multiple Allele.

Definition.

All such altered alternative forms of a gene, whose number is more than two, are called multiple alleles.

Cause.

Gene mutations may produce many different alleles of a gene. Some genes may have as many as 300 alleles.

In an individual.

Any two of these multiple alleles can be present in the genome of a diploid organism, but a haploid organism or a gamete can have just one of them in its genome. ABO – The First Discovered Multiple Allelic Blood Group System in Man Discovery. ABO blood group system was discovered by Karl Landsteiner in 1901.

Phenotypes.

ABO system has four different phenotypes which are distinct from each other on the basis of specific antigens on the surface of RBC.

 A.A person having antigen A has blood group A;

 B. a person having antigen B has blood group B;

 AB. a person having both the antigens A and B has blood group AB;

 O. a person having neither antigen A nor B would have blood group O.

Genotypes. Bernstein explained the genetic basis of ABO system in 1925. Gene. This blood group system is encoded by a single polymorphic gene I on chromosome 9.

Alleles.

It has three multiple alleles I A , IB , and

i. Allele IA specifies production of antigen A, and allele IB specifies production of antigen

B,

but allele i does not specify any antigen. Dominance relations. Their dominance relations are interesting too. Alleles I A and I B are codominant to each other, because each expresses equally in I A and I B heterozygote to produce AB phenotype. But allele i is recessive to both I A and I B . Therefore.

I A I A or IA i genotypes will produce phenotype A. Similarly I B I B or I B i produces phenotype B. The homozygous ii will produce phenotype O.

Gene Expression.

The blood group alleles start their expression at early embryonic stage and keep on expressing themselves till death. Therefore, the blood group phenotype of a person never changes throughout life.

Antibodies Production.

Anti-A and anti-B antibodies appear in plasma during the first few months after birth. They are naturally occurring in the absence of corresponding antigen.

A phenotype. T

he blood serum of A phenotype contains anti-B antibodies. They will agglutinate or clump any RBC which have B antigens on them. B phenotype contains anti-A antibodies in the serum and agglutinate any RBC with antigen A. Phenotype AB has neither anti-A nor anti-B antibodies in the serum.

O phenotype.

The serum of O blood type contains both anti-A and anti-B antibodies. Antiserum. The blood serum containing anti-bodies is called antiserum