Multiple Alleles Notes

Alleles, also known as allelomorphs, are the different forms of a gene located at the same position on homologous chromosomes. In some cases, multiple alleles exist for a single gene, meaning a diploid cell will contain two alleles (one from each homologous chromosome) and a haploid cell will contain only one allele.

When a trait is governed by multiple alleles, it is called multiple allelism.

Multiple Alleles Characteristics

  • Can produce more than two phenotypes
  • Occurs when more than two alleles exist for a single gene
  • Each allele has an equal chance of being inherited by offspring

Multiple alleles of a gene can be found at the same locus on the chromosome.

Multiple alleles are alternative forms of the same gene, which can influence the same trait.

The mutant alleles are mostly recessive to the wild-type allele.

The wild type is considered the standard, and all other alleles are considered variants.

The variant or mutant allele may have a dominant, recessive, or intermediate phenotypic effect.

An individual can have only two alleles for the given gene, but multiple alleles exist at the population level.

An individual can have either one type of allele (homozygous) or two different alleles (heterozygous).

A series of mutant alleles can exist in a population, generated by a spontaneous mutation of the wild type and the mutant alleles.

Multiple alleles can result in a range of phenotypic traits for a character in a population.

Multiple Alleles Examples

The coat colour of rabbits, the A, B, AB and O blood groups in humans, and the eye colour in Drosophila are all examples of multiple alleles.

  1. Blood Groups in Humans

The gene I, which determines the human blood group types A, B, AB and O, exists in three allelic forms: IA, IB and i. The allele IA produces surface antigen A on the red blood cells, the allele IB produces surface antigen B on the red blood cells, and the allele i specifies no antigen. IA and IB are codominant over allele i.

If a person has two IA alleles (IAIA), their blood group is A. If a person has one IA and one i allele (IAi), their blood group is also A. A person with two IBIB alleles has a 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 Rabbits

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.

A rabbit with the genotype CC has either black or brown fur, cchcch has either chinchilla or greyish coat colour, chch has a Himalayan pattern and cc has a white coat colour, i.e. albino. The four common alleles of the C gene are C, cch, ch and c.

Due to multiple alleles, there are many possible combinations of alleles.

The black C allele is dominant over the other three alleles.

The Cch allele exhibits incomplete dominance when compared to the Himalayan and Albino alleles.

The Himalayan (ch) allele is dominant over the albino (c) allele.

3. Eye Colour in Drosophila

In Drosophila, eye colour is determined by multiple alleles, with a dozen different alleles displaying phenotypes between the wild-type red colour (w+) and white colour (w). These alleles include coral, blood, eosin, cherry, apricot, and more. The wild-type red colour is dominant over all other mutant alleles, while the white colour is recessive to all the alleles.

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