Alleles or allelomorphs are the alternative forms of a gene present at the same locus on the hom*ologous 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 hom*ologous chromosomes and a haploid cell contains only one allele.
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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 (hom*ozygous) 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 hom*ologous 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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FAQs
Multiple allelism, on the other hand, occurs when more than two gene forms exist for the same locus. For example, one allele of the gene-producing height can be tall, while the other allele can be dwarf. It simply indicates that for a given gene, three or more alleles are present in the population.
What if there are 4 multiple alleles of a gene? ›
If there are 4 alleles for a gene, then what would be the number of genotypes? Explanation: The number of genotypic expression in multiple allele series is given by the formula 0.5[n(n+1)], where n is the number of alleles. So here it would be 10.
How do you find the number of multiple alleles? ›
The formula to find the number of genotypes for multiple allelism is (n/2)(n+1) where n is the number of alleles. So, the correct option is '(c) At the same locus is one type of chromosomes'. Note: - The term 'multiple alleles' is referred to as the presence of three or more alleles for a particular gene.
How is the ABO blood group related to multiple alleles? ›
The gene responsible for the ABO blood group system is I. This gene exists in three allelic forms - IA, IB and i. IA and IB are dominant alleles and i is the recessive allele. Since, three alleles for ABO blood group are found in a population, it is an example of multiple allelism.
What examples best define multiple alleles? ›
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.
Why are multiple alleles important? ›
Although humans (and all diploid organisms) can have only two alleles for any given gene in genetics, multiple allele traits may exist at the population level. Thus, multiple alleles are important in promoting variation within the same species.
How many alleles per gene? ›
When the copies of a gene differ from each other, they are known as alleles. A given gene may have multiple different alleles, though only two alleles are present at the gene's locus in any individual.
What trait is multiple alleles? ›
There are many traits controlled by multiple alleles. In humans, this includes hair color, eye color, and blood type. Though it may seem like there are only a few possible hair colors, there are actually many variations in hue and shade owing to a person's genes.
Which blood type is controlled by three alleles? ›
The four main blood groups A, B, AB, and O are controlled by three alleles: A, B, and O. As humans are diploid, only two of these can be present in any one genotype.
Is eye color multiple alleles? ›
Yes, eye color is an example of multiple alleles. There are more than 2 alleles that encode for the same. In multiple allelism, different forms of the same gene exist in a population.
An allele of a gene is said to be dominant when it effectively overrules the other (recessive) allele. Eye colour and blood groups are both examples of dominant/recessive gene relationships.
How many alleles in a gene pair? ›
Some genes have a variety of different forms, which are located at the same position, or genetic locus, on a chromosome. Humans are called diploid organisms because they have two alleles at each genetic locus, with one allele inherited from each parent. Each pair of alleles represents the genotype of a specific gene.
What is the law of multiple alleles? ›
With multiple alleles, that means there are more than two phenotypes available depending on the dominant or recessive alleles that are available in the trait and the dominance pattern the individual alleles follow when combined together.
How are ABO alleles inherited? ›
The ABO blood group antigens are encoded by one genetic locus, the ABO locus, which has three alternative (allelic) forms—A, B, and O. A child receives one of the three alleles from each parent, giving rise to six possible genotypes and four possible blood types (phenotypes).
What is meant when a gene is said to have multiple alleles? ›
An allele is one of two or more specific variants of a gene that exists at the same locus on a chromosome; a single allele has only one variant of a gene, while multiple alleles have more than two alleles for a gene, such in the case of blood type.
Which of the following best define multiple alleles? ›
The correct answer to “Which of the following best defines multiple alleles for a given trait?” is answer choice (A). Multiple alleles refer to three or more alleles occupying the same locus.
What is the definition of a multiple allele quizlet? ›
Multiple Alleles. - When three or more alleles control a trait. Example. Blood Types in humans.
What is an example of a multiple allele flower? ›
Multiple Alleles in Plants
There are at least four alleles in the multiple allele system that determines flower colour in snapdragons: W (white), R (red), P (pink), and I (intermediate). Plants with two copies of the W allele have white flowers because it is dominant over all other alleles.
