Multiple Allelism can be defined as the presence of numerous alleles controlling more than two alternative opposing features at a single genetic locus. Have you ever imagined why we have different types of blood groups? Why do cats, rabbits and dogs have so many different coat colours? This arises because of multiple allelism. Features of alleles, Multiple alleles express various alternatives to one trait, the same genes have more than two alleles are few features of alleles.

Multiple alleles exist in populations where many variations of a single gene are present. In this article, we will know more about the multiple alleles definition multiple alleles example, and more.

Multiple Alleles Definition

What is the multiple alleles definition in biology? When a character is controlled by three or more alleles for a gene, it is called multiple alleles, and the phenomenon is called multiple allelism. Such multiple alleles are responsible for producing different types of phenotypes and genotypes. Multiple allelism can be observed only during population studies.

Define Multiple Alleles: Development in a Population

1. In diploid organisms, each organism has the ability to express two alleles at the same time.
2. The alleles can be either the same (called homozygous genotype) or different (called heterozygous genotype).
3. In a haploid organism, for a gene, only one allele is present. However, when the population study is carried out for the same genes, multiple alleles for the same gene can be observed.
4. New alleles are created by a spontaneous mutation in both haploid and diploid organisms.
5. Such creation of new allelic variants also leads to the development of variation in phenotypes of the population by changing the sequence of amino acids, either in a simple or drastic way.
6. Mutations may lead to the production of variant forms of alleles, leading to the multiple allelism phenomena.
7. Multiple Alleles combine in different ways in a population which produces different kinds of phenotypes. These phenotypes are produced by the proteins encoded for various alleles.
8. Even though each gene produces its effect for the same character, the presence of different allelic forms leads to variation in the structure and functions of these proteins, which develop different phenotypes or traits.
9. Thus, multiple alleles lead to the formation of variants in the phenotypic trait of an organism.
10. When many alleles exist for the same gene, the most common phenotype or genotype in the natural population is denoted as the wild type (or \(‘ + ‘\)), and all other phenotypes or genotypes are considered as variants or mutants derived from the wild type.
11. The variants may be recessive or dominant to the wild type allele.

Fig: Multiple Allelism in Dogs

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Number of Genotype \(= \frac{n}{2}\;x\;n + 1\)
where \(n = \) number of alleles.

Multiple Allelism Characteristics

1. Multiple allelism can be observed only when population studies are carried out.
2. Multiple alleles are situated on homologous chromosomes at the same locus.
3. They influence one or the same character but result in the formation of various traits.
4. When any two of the multiple alleles are crossed, the phenotype is a mutant type and not the wild type.
5. They show the typical monohybrid ratio for a particular character in the \({F_2}\) generation if the alleles are not codominant.

Multiple Alleles Examples

Multiple alleles express different alternatives to a single trait. Different alleles may show codominance, dominance-recessive behaviour or incomplete dominance among themselves. Following are the different examples of multiple allelism:

ABO Blood Group System in Humans

a. The \(ABO\) Blood group system is one of the examples of multiple allelism where there are three alleles present in the population, i.e., \({I^A},\,{I^B}\) and \(i.\)
b. Where \({I^A} = \) codes for the antigen \(A\) present on the surface of \(RBCs.\)
 \({I^B} = \) codes for the antigen \(B\) present on the surface of \(RBCs.\)
\(i = \) codes for no antigens on the surface of \(RBCs.\)
c. \({I^A} \) and \({I^B}\) alleles are codominant with each other and are both dominant over the \(‘i’\) allele.
d. Even though there are three alleles present in a population, each individual gets only two of the alleles from their parents.
e. There are six different genotypes and four different phenotypes for the three alleles of the blood group.
f. The number of possible phenotypes depends on the dominance relationships between the three alleles.

Fig: Inheritance of ABO Blood Group System in Humans

Genetic Basis of ABO Blood Groups in Humans

1. In humans, the blood groups are controlled by a single gene, \({\rm{‘I’}}\) (\({\rm{I}}\) represent isohemagglutinin), which represents the production of glycoprotein or antigen present on the surface of the \(RBCs.\)
2. The blood groups are represented by a set of three allelic expressions, namely, \({\rm{‘I’,’I’}}\) and \(‘{\bf{i}}’.\) Any individual will carry two of these three alleles. As three alleles of a gene are involved in the \(ABO\) blood grouping system, it is a great example of multiple allelism, i.e., a character controlled by more than two alleles.
3. The allele \(I\)^A produces a glycoprotein \(A\) or antigen \(A\); in particular, it produces N-acetylgalactosaminyltransferase enzyme, which recognizes \(H\) antigen present in the \(RBC\) membrane and adds N-acetylgalactosamine to the sugar part of \(H\) antigen to form \(A\) antigen.
4. The allele \(I\)^B produces a glycoprotein \(B\) or antigen \(B\); in particular, it produces Beta-galactosyltransferase enzyme, which recognizes \(H\) antigen present in the \(RBC\) membrane and adds Beta-galactose to the sugar part of \(H\) antigen to form \(A\) antigen. 
5. But the allele \(‘{\bf{i}}’\) does not produce any enzyme and thus no specific antigen or glycoprotein. 
6. The allele \(I\)^A is dominant over \(‘{\bf{i}}’\), and allele \(I\)^B is also dominant over \(‘{\bf{i}}’.\)
7. Allele \(‘{\bf{i}}’\) is the recessive allele.
8. When the alleles I^A and I^B are together, they are codominant, and both the glycoprotein \(A\) and \(B\) are produced.
9. Hence, the blood group is determined by the presence or absence of one or both the blood glycoproteins, i.e., group \(A\) has glycoprotein \(‘A’\), group \(B\) has glycoprotein \(‘B’\), group \(AB\) has both the glycoproteins, while group \(O\) has neither of them.

Blood Group	Gene Combination	Sugar moiety added to form glycoprotein
\(A\)	\({I^A}{I^A};{\rm{ }}{I^A}i\)	\(N – \)acetylgalactosamine
\(B\)	\({I^B}{I^B};{\rm{ }}{I^B}i\)	Beta-galactose
\(AB\)	\({I^A}{I^B}\)	\(N – \)-acetylgalactosamine,Beta-galactose
\(O\)	\(i{\rm{ }}i\)	No sugar

Table:  Blood Groups with their Gene Combinations and Sugar Moiety on the Surface on RBCs.

Eye Colour in Fruit Flies

The common fruit flies Drosophila melanogaster has only four chromosomes, of which around \(17,000\) genes exist.
b. Each gene controls a different character of the fly that can undergo mutations resulting in the formation of new alleles and thus new traits.
c. For example- the gene for eye colour determines whether the fly will have orange/brown, red, sepia or white eyes. Both the orange, sepia and white alleles are recessive to the wild type red eye colour.

Fig: Different Types of Flies of the Same Species Drosophila melanogaster

Multiple Allelism: Importance

The importance of multiple allelism is as follows:
1. Multiple Allelism leads to the formation of a number of variants or mutants.
2. It helps in the occurrence of the natural selection process.
3. It helps in increasing our knowledge of heredity.

Multiple Alleles Biology Discussion – Summary

Thus, when a character is controlled by three or more alleles for a gene, it is called multiple alleles, and the phenomenon is called multiple allelism in biology. Multiple allelism has led to the idea that different amounts of heterochromatin prevent the genes to different degrees. This condition leads to the formation of a number of variants or mutants that helps in the natural selection process leading to evolution. Each newly mutated allele adds a new combination to the almost infinite pool of genetic variation.

FAQs on Multiple Allelism

Following are some of the frequently asked questions on multiple alleles biology discussion that students must know:

Q.1. What is the importance of Multiple Allelism?
Ans: The importance of Multiple Allelism are as follows:
a. Multiple Allelism leads to the formation of a number of variants or mutants.
b. It helps in the occurrence of the natural selection process.
c. It helps in the process of evolution also.
d. It helps in increasing our knowledge of heredity.

Q.2. How many Multiple Alleles are there?
Ans: Different individuals in a population may have different pairs of these Multiple Alleles, though a diploid organism can only have two alleles for a particular trait or gene.

Q.3. Are Multiple Alleles common in humans?
Ans: Yes, Multiple Alleles are common in most diploid organisms, including humans.

Q.4. What does it mean when there are Multiple Alleles for a trait?
Ans: It means that there are many variations present for a single gene.

Q.5. What is the formula to calculate the number of genotypes in the case of multiple alleles?
Ans: Number of Genotype \(= \frac{n}{2}\;x\;n + 1,\) where \(n =\) number of alleles.

Q.6. What is Multiple Allelism? Give an example.
Ans: Multiple Allelism is a condition in which three or more alleles of a particular gene are present on the same homologous chromosome, producing different types of phenotypes and genotypes.
For e.g., the \(ABO\) blood group system in humans.

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