Brief note on Multiple Alleles

Mendelian laws were the central part of the genetics that contributed significantly in shaping the hereditary patterns of genes. However, some limitations were found due times.  After rediscovery of Mendel’s law of inheritance, various geneticist conducted experiments & obtain results which were different from Mendel’s laws. One such exception is applied in the case of multiple allelism. Where any character is controlled by more than two alternating forms of genes, alleles.

It is now well established that all characters of as individual is not controlled by two alternative forms of genes or alleles, rather through several alternative forms or alleles.

When any of the three or more allele forms of a gene occupy the same locus in a given pair of homologous chromosomes, they are said to constitute a series of multiple alleles.

Multiple alleles exhibit distinguished features like:

i)             Multiple allele always occupy same locus is the chromosome.

ii)           Always influence the same character.

iii)          No crossing over occupy b/w then

iv)          Of the series, the wild type allele is mostly dominant while other are usually, not always recessive.

Example –  Rabbit’s coat colour                  

                   Eye colours & wing sizes of Drosophila spp.

                   Human ABO blood groups

 

 

Explanation:

Bernstein (1925) proposed that inheritance of A, B, AB & O blood types in human is determined by three allelemorphic genes instead of biallelic system. The genes controlling blood type has been labelled as L (for its discovered Landsteiner) or I (iso-agglutinin).  

Thus, I gene exists in 3 different forms – i) I^A (A antigen)

 ii) I^B (B antigen)

iii) I^O (no antigen)

Beside these allelic variations, I^A gene exhibit different subgroups & exist in at least four allelic forms, I^A1, I^A2,I^A3 and I^A4 at the dominance hierarchy I^A1> I^A2>I^A3 > I^A4 .

 

Allelic configuration	Type of Blood	A antigen present	B antigen present	Antibody present
IA IA / IA IO	A	+	-	Anti B (β)
IB IB / IB IO	B	-	+	Anti A (α)
IA IB	AB	+	+	None
IO IO	O	-	-	Anti A (α) Anti B (β).