Multiple allele

 Mendel implied that only two alleles, one dominant and one recessive, could exist for a given gene. Alleles are alternative forms of a gene, and they are responsible for differences in the phenotypic expression of a given trait (e.g., brown eyes versus green eyes). Although individual humans (and all diploid organisms) can only have two alleles for a given gene, multiple alleles may exist at the population level such that many combinations of two alleles are observed.

Note that when many alleles exist for the same gene, the convention is to denote the most common phenotype or genotype among wild animals as the wild type (often abbreviated “+”); this is considered the standard or norm. All other phenotypes or genotypes are considered variants of this standard, meaning that they deviate from the wild type. The variant may be recessive or dominant to the wild-type allele.

Thus, more than two kinds of alleles occupying the same locus in the individual chromosomes are referred to as multiple alleles. In short many alleles of a single gene are called multiple alleles.

The word allele is a general term to denote the alternative forms of a gene or contrasting gene pair that denote the alternative form of a gene is called an allele. These alleles were previously considered by Bateson as a hypothetical partner in Mendelian segregation. Dawson and Whitehouse in England proposed the term parallel for all the gene mutations at a given locus in a chromosome. These differ from the multiple factors in one respect that multiple factors occupy different loci while alleles occupy the same locus.

Characteristics of multiple alleles

1. The study of multiple alleles may be done in the population.

2. Multiple alleles are situated on homologous chromosomes at the same locus.

3. Multiple alleles always control the same character of an individual. However, the expression of the character will differ depending on the allele present.

4. The wild-type (normal) allele is nearly always dominant while the other mutant alleles in the series may show dominance or there may be an intermediate phenotypic effect.

5. There is no crossing over between the members of multiple alleles. Crossing overtakes place between two different genes (inter-generic recombination) and does not occur within a gene (intragenic recombination).

6. Multiple alleles never show complementation with each other. By complementation test, them allelic and non-allelic genes may be differentiated well. The production of wild-type phenotype in a trans-heterozygote for 2 mutant alleles is known as the complementation test.

7. When any two of the mutant multiple alleles are crossed, the phenotype is of a mutant type and not the wild type.

8. Further, F2 generations from such crosses show a typical monohybrid ratio for the concerned character.

Test for allelism

There are two types of tests that are used for allelism, viz., recombination test and complementation test.

1. Recombination test

Earlier it was believed that recombination can occur between two genes but not within a gene. Thus, if a cross between two mutants says m1m1 and m2m2 produce wild type in test cross or in F2 then m1 and m2 are considered as non-allelic because the production of the wild type is not possible without recombination. If no wild type appears in test cross or F2 then m1 and m2 are considered as allelic forms. Now intragenic recombination has been reported in many organisms. Hence this concept is no more valid.

2. Complementation Test

Complementation refers to the appearance of a wild phenotype when two mutants are crossed. A complementation test is used to determine whether two mutant alleles belong to the same gene or two different genes. If there is complementation, the mutants are located in different genes, otherwise, they are located in the same gene.

Alleles may be arranged in two ways, viz., cis position and transposition. When two wild alleles are located in one chromosome and their mutant alleles are in homologous chromosomes (++/ab), it is known as cis-arrangement. Thus, in the cis position alleles are linked in the coupling phase. On the other hand, when one wild and one mutant type alleles are located in each homologous chromosome (+a/+b), it is known as the transposition or repulsion phase of alleles.

Oliver in 1940 first demonstrated that intragenic recombination occurred in the Lozenge gene of Drosophila. The two mutant alleles are considered to belong to the same gene if their cis heterozygotes produce wild type and trans-heterozygotes lead to mutant type. If both trans and cis heterozygotes lead to the development of wild type, the mutant alleles are located in two different genes. Thus, the cis-trans test is a more reliable test of allelism.