Alleles or allelomorphs are the alternative forms of a gene present at the same locus on the homologous chromosomes. Some genes have more than two allelic forms, which is referred to as multiple alleles. A diploid cell contains any of the two alleles, one each on the homologous chromosomes and a haploid cell contains only one allele.
When a trait is governed by multiple alleles it is called multiple allelism
Multiple Alleles Characteristics
- Multiple alleles of a gene occupy the same locus on the chromosome.
- Multiple alleles are the alternative forms of the same gene so they influence the same trait.
- The wild-type allele is mostly dominant over the mutant alleles.
- The wild type is considered the standard and all other alleles are considered variants.
- The variant or mutant allele may be dominant, recessive or have an intermediate phenotypic effect.
- Multiple alleles exist at the population level and an individual can have only two alleles for the given gene.
- An individual can have only one type of allele (homozygous) or two different alleles (heterozygous).
- A series of mutant alleles can exist in a population. They are generated by a spontaneous mutation in the wild type and the mutant alleles.
- Multiple alleles lead to the formation of varied phenotypic traits for a character in the population.
Multiple Alleles Examples
The most common examples of multiple alleles are the coat colour of rabbits, A, B, AB and O blood groups in humans and the eye colour in Drosophila.
1. Blood Groups in Humans
The A, B, AB and O blood group types in humans are determined by three alleles of a gene. The gene I, which determines the blood type, exists in three allelic forms, which are IA, IB and i. The allele IA produces surface antigen A on the red blood cells and the allele IB produces the surface antigen B on the red blood cells and the allele i specifies no antigen. Allele IA and IB are codominant over allele i.
A person has any of the two alleles out of the three alleles on the homologous chromosomes. If a person has two IA alleles (IAIA) or one IA and one i allele (IAi) then the blood group is A group. Similarly, IBIB and IBi give rise to B blood type, IAIB gives rise to AB blood type, and two i alleles (ii) give rise to O blood type.
2. Coat Colour in Rabbit
The coat colour of rabbits is determined by the C gene. The C gene has multiple allelic forms that lead to varied phenotypes. The different phenotypes for coat colour in rabbits are agouti (full colour), chinchilla, Himalayan and albino.
The four common allele of C gene are C, cch, ch and c. A rabbit with genotype CC has black or brown fur, cchcch have chinchilla or greyish coat colour, chch have Himalayan pattern and cc have white coat colour, i.e. albino.
Due to multiple alleles, there are many combinations of alleles possible.
- The black C allele is dominant over the other three alleles.
- The cch (chinchilla) allele shows incomplete dominance over Himalayan and albino alleles.
- The ch (Himalayan) allele shows dominance over the albino allele (c).
3. Eye Colour in Drosophila
The eye colour in Drosophila is another trait that is determined by multiple alleles. There are a dozen different alleles that show phenotype between the wild-type red colour (w+) and white colour (w). E.g. coral, blood, eosin, cherry, apricot, etc. The wild-type red colour is dominant over all other mutant alleles and the white colour is recessive to all the alleles.
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