Multiple alleles

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 MULTIPLE ALLELES – LONG ANSWER


In classical Mendelian genetics, each character is controlled by two alternative forms of a gene, known as alleles. However, later studies showed that a single gene may have more than two allelic forms in a population. Such a condition is known as multiple allelism, and the alleles are called multiple alleles.


⭐ Definition


Multiple alleles are three or more alternative forms of the same gene that occupy the same locus on a homologous pair of chromosomes, but only two alleles can exist in a diploid organism at a time.


In simple words:

Many alleles exist in the population, but each organism still carries only two of them.


⭐ Characteristics of Multiple Alleles


Multiple alleles arise due to mutation of the original gene.


They occupy the same gene locus, so they are mutually exclusive—only one allele can be present on one chromosome.


A single individual can have only two alleles, but a population may have many.


They show a hierarchy of dominance (one allele may dominate over others).


They increase genetic variation in a population.


⭐ Most Important Example: ABO Blood Group System (Human)


The ABO blood group in humans is the best-known example of multiple alleles.


It is controlled by the gene I (isoagglutinin), which has three allelic forms:


Iᴬ — produces A antigen


Iᴮ — produces B antigen


i — produces no antigen


Dominance Relationships


Iᴬ and Iᴮ are codominant

→ Both express equally when present together


Iᴬ and Iᴮ are dominant over i

→ i is recessive


Possible Genotypes and Phenotypes

Genotype Phenotype (Blood Group)

IᴬIᴬ, Iᴬi A

IᴮIᴮ, Iᴮi B

IᴬIᴮ AB

ii O

⭐ Why ABO is a Multiple Allele System?


Because:


The gene exists in three forms (Iᴬ, Iᴮ, i) in the human population.


But each person carries only two of them (e.g., IᴬIᴮ, Iᴮi, ii).


Thus, the ABO system perfectly demonstrates the concept of multiple allelism.


⭐ Another Example: Coat Colour in Rabbits


The coat colour of rabbits is controlled by a gene with four multiple alleles:


C – full colour (dominant)


cᶜʰ – chinchilla (grey)


cʰ – Himalayan (white with black extremities)


c – albino (recessive)


Hierarchy of dominance:

C > cᶜʰ > cʰ > c


This gives rise to several combinations and phenotypes.


⭐ Genetic and Evolutionary Significance of Multiple Alleles


Increase genetic diversity in populations.


Help in better adaptation to environmental changes.


Provide deeper understanding of inheritance patterns beyond simple Mendelian ratios.


Useful in blood transfusion, forensics, and population studies (e.g., ABO system).


⭐ Conclusion


Multiple alleles represent an extension of Mendelian genetics where more than two allelic forms of a gene exist in a population. The ABO blood group system in humans is the classic example, demonstrating how three alleles—Iᴬ, Iᴮ, and i—interact through dominance and codominance. Multiple alleles increase genetic variation and have important biological and medical significance.

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