Multiple Alleles: ABO Blood Group System and Coat Colour in Rabbit

The presence of more than two alleles in a single gene in different combination is called multiple alleles. Normally two alleles determine the traits in one individual. One allele is carried on each chromosome of a homologous pair. Some populations have many different alleles. They can transfer these alleles to any member of the population. These are called multiple alleles.

Genes for a particular trait are present at the same position on a chromosome. The position of gene on the chromosome is called its locus.

Examples of Multiple Alleles:

1.           Coat Colour in Rabbit:

Coat colour in rabbits is a classic example of a gene with multiple alleles. The colour determining gene is denoted by lowercase letter ‘c’. It has four alleles. Three of which are distinguished by a superscript. These four alleles are:

(i) c (Albino), (ii) c^h (Himalayan), (iii) c^ch (Chinchilla), (iv) c⁺ (Wild-type)

In homozygous condition, each allele has a characteristic effect on the coat colour.

Rabbit Colour	Genotype	Phenotype
Albino	cc	White hairs over the entire body
Himalayan	chch	Black hairs on the extremities; white hairs everywhere else
Chinchilla	cchcch	White hairs with black tips on the body
Wild-type	c+c+	Coloured hairs over the entire body

 

2.           ABO Blood Group System:

Karl Landsteiner discovered ABO blood group system in 1901. ABO system has four different phenotypes. These groups are different from each other on the basis of antigens. These antigens are present on the surface of RBCs.

Bernstein explained the genetic basis of ABO system in 1925. This blood group system has a single polymorphic (with many forms) gene I on chromosome 9. This gene has three multiple alleles I^A, I^B and i.

Blood Type	Genotype	Antigens on RBC	Antibodies
A	IA IA (homozygous) IA i (heterozygous)	A	Anti-B
B	IB IB (homozygous) IB i (heterozygous)	B	Anti-A
AB	IA IB (heterozygous)	AB	None
O	i i (homozygous)	None	Anti-A and Anti-B

• The alleles I^A and I^B are codominant to each other. So, they are expressed equally in I^A I^B heterozygote to produce AB phenotype.
• Allele i is recessive to both I^A and I^B. Therefore, I^A I^A or I^A i genotype will produce phenotype A.
• Similarly, I^B I^B or I^B i produce phenotype B.
• The homozygous ii will produce phenotype O.

Antibodies of Blood Groups:

The blood group alleles start their expression at early embryonic stage. They keep on expressing till death. Therefore, the blood group phenotype of a person never changes throughout life. Anti-A and anti-B antibodies appear in plasma during the first few months after birth. These antigens occur naturally in the absence of corresponding antigen.

• The blood serum of a phenotype A contains anti-B antibodies. They agglutinate or clump any RBC with B antigens.
• B phenotype contains anti-A in the serum. It agglutinates any RBC with antigen A.
• The Phenotype AB has neither anti-A nor anti-B antibodies in the serum.
• The serum of O blood type contains both anti-A and anti-B antibodies.

Transfusion of Blood:

Safe blood transfusion does not cause agglutination in the recipient. The clumped cells cannot pass through fine capillaries. Therefore, agglutination causes serious results. The blood samples of the donor and the recipient are cross-matched for compatibility. Dangerous hemolytic reaction occurs in incompatible transfusion. The antibodies of the recipient can destroy the RBC of donor. Similarly, the antibodies of the donor can destroy the RBC of the recipient.

Following blood groups are used for safe blood transfusion:

·         Blood group A: Blood group A can be transfused only into A. The blood group AB does not have anti-A antibodies. So, AB is also recipients of group A.

·         Blood group B: Blood group B can be transfused only into B. The blood group AB does not have anti-B antibodies. So, AB is also recipients of group B.

·         Blood group AB: The AB group does not have anti-A and anti-B antibodies. Thus AB blood can be transfused only into AB recipients. AB blood group individuals can receive transfusions of blood from any of the four blood groups. So, they are called universal recipients.

·         Blood group O: Blood group O does not have A or B antigen. But it has anti-A and anti-B antibodies. Thus an O recipient can only be given transfusion from a donor O. The donor's antibodies are quickly absorbed by other tissues. Or they are greatly diluted in the recipient's blood. So, phenotype O can also be used as donor for small transfusions to A, B and AB. Thus O blood group is called universal donors.

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