Multiple Allele

Multiple Allele

An allele is the alternative form of a gene. When more than two forms of a gene exist on a single locus of a chromosome, the alleles are then called as multiple alleles.

Formation of multiple alleles:

Multiple alleles are formed by gene mutation. A slight change in the nucleotide sequence of a gene results in the formation of alleles.

Number of multiple alleles: 

The number of alleles controlling a character varies. The ABO blood group system is controlled by three alleles of gene I. Some genes may have as many as 300 alleles for a character. Multiples alleles exist in the individual of a population, but individuals have only two of those alleles. It is because most of organisms are diploid having two homologs of each chromosome.

ABO BLOOD GROUP:

History

ABO blood group system was discovered in 1901 by Karl Landsteiner of the University of Vienna. Later he was awarded a Nobel Prize. ABO blood groups are found in all humans and in many primates such as apes, chimpanzees, baboons and gorillas.

Genetic Basis

The ABO blood groups are controlled by Gene I located on the chromosome 9. Gene I has three allelic forms i.e. I^A ,I^B and i. The I^A and I^B alleles each encode a glycosyltransferase that catalyzes the synthesis of the A and B antigen, respectively. The O or i allele encodes an inactive glycosyltransferase that leaves the ABO antigen precursor (the H antigen) unmodified.

 

Blood type	Antigen on RBC	Allele for Antigen	Possible Genotypes	Dominance relation
Type A	Antigen A	IA	IA IA ,  IAi	IA Dominant to i
Type B	Antigen B	IB	IB IB , IBi	IB Dominant to i
Type AB	Antigen A & B	IA and IB	IA IB	IA and IB  Co-Dominant
Type O	None	i	ii	i is recessive

 

Blood group types

A person’s blood group may be one of four types: A, B, AB, or O. These blood group types are due to the presence of antigen A and antigen B on the surface of RBC. A person having antigen a on the surface of RBC will have blood group A, having antigen B on the surface of RBC will have blood group B, having both A and B antigens will have blood group AB. When no antigen is expressed (nor A neither B), then the blood group is said to be O blood group.

Ability to produce Antibodies against antigen A and Antigen B

Blood group is determined by antigens present on the surface of RBCs. The immune system has the ability to produce antibodies against the foreign antigens. In normal conditions, the body do not produce antibodies against body own cells but can produce against any foreign agent that enters into the body.

Blood group A: An individual with the blood groups A has antigen A on the RBC surface, therefore, it only produces Antibodies against the antigen B.

Blood group B: A person having blood group B produces antibodies against the antigen A.

Blood group O: A person having none of antigen on the surface of RBC (Blood group O) produces anti-A and Anti-B antibodies.

Blood group AB: Those have AB blood group do not produce antibodies.

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