Extensions of Mendelian Genetics (Deviation from Mendelism) - Multiple Alleles

Multiple alleles:

A given phenotypic trait of an individual depends on a single pair of genes, each of which occupies a specific position called the locus on homologous chromosome. When any of the three or more allelic forms of a gene occupy the same locus in a given pair of homologous chromosomes, they are said to be called multiple alleles.

Characteristics of multiple alleles:

1. Multiple alleles of a series always occupy the same locus in the homologous chromosome. Therefore, no crossing over occurs within the alleles of a series.
2. Multiple alleles are always responsible for the same character.
3. The wild type alleles of a series exhibit dominant character whereas mutant type will influence dominance or an intermediate phenotypic effect.
4. When any two of the mutant multiple alleles are crossed the phenotype is always mutant type and not the wild type.

Examples:

1. ABO blood grouping also provides a good example of 'multiple alleles'.
2. ABO blood group system in humans is an illustration of multiple allelism because gene i has three allelic forms, I^A, I^B, and i which means that three alleles control the same trait i.e., blood group.
   

   Table showing the Genetic Basis of Blood Groups in Human Population
   Father		Mother		Children
   Phenotype	Genotype	Phenotype	Genotype	Phenotype
   A	IAIA	A	IAIA	A, O
   	IAi		IAi
   B	IBIB	B	IBIB	B, O
   	IBi		IBi
   A	IAIA	B	IBIB	A, B, AB, O
   	IAi		IBi
   A	IAIA	O	ii	A, O
   	IAi
   B	IBIB	O	ii	B, O
   	IBi
   AB	IAIB	A	IAIA	A, AB, B
   			IAi
   AB	IAIB	B	IBIB	B, AB, A
   			IBi
   AB	IAIB	O	ii	A, B
   AB	IAIB	AB	IAIB	A, AB, B
   O	ii	O	ii	O

3. Occasionally, a single gene product may produce more than one effect.
   For example, starch synthesis in pea seeds is controlled by one gene. It has two alleles (B and b).
   Genotype - BB (homozygous) synthesizes starch effectively, hence larger starch grains are produced.
   Genotype - bb (homozygous) synthesizes starch less effectively, therefore smaller starch grains are produced.
4. After maturation of the seeds, BB seeds are round and the bb seeds are wrinkled.
5. Heterozygotes produce round seeds, and so B seems to be the dominant allele. But, the starch grains produced are of intermediate size in Bb seeds. So if the starch grain size is considered as the phenotype, then from this angle, the alleles show incomplete dominance.
6. Therefore, Dominance is not an autonomous feature of a gene or the product that it has information for. It depends as much on the gene product and the production of a particular phenotype from this product as it does on the particular phenotype that we choose to examine, in case more than one phenotype is influenced by the same gene.