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Alleles – Introduction, Definition With Examples

Class 8
May 9, 2023

Introduction of Alleles

An allele is an important term in genetics. Gene is a structural unit of chromosomes; genes carry heredity from generation to generation. The alleles are pairs of genes located in a specific area of the chromosome. First, let us discuss the meaning of alleles.


Definition- alleles are a pair of genes that occupy a location on a chromosome and control the same trait.

In the human chromosome, alleles are present in pairs and maintain the same trait. Therefore, humans are diploid organisms. Two similar alleles are present in the genetic locus, where one allele is inherited from each parent. An allele can be two or more variants of a gene at one genetic locus.

Meaning of Alleles

The word allele comes from the Greek word ‘allows.’ The word ‘allos’ means other. An allele means various varieties of a gene. The alleles present in a genetic locus maintain the same trait. However, alleles are present in a locus as a set or pair; they can be found in more than two numbers. In the human chromosome, alleles are present in a pair only to carry the heredity.



Genotype of Allele

Alleles are present in a particular location of a chromosome. The chromosome is the central part of an organism. Alleles are present in an organism that builds a genotype. Genotype is of two types depending on the similarities of the alleles.

When the pair of alleles are the same, it is a homozygous genotype. When the pair of alleles in a location are not the same, it is heterozygous genotypes. In the homozygous genotype, the allele is not dominant or recessive. But the heterozygous genotype includes one dominant allele. The dominant allele rules the features of the recessive allele.


Example of Alleles

Example of a pea plant. The alleles showing the color of the flower build heterozygous genotype, the purple allele is dominant, and the white is recessive. The height of a plant, tall is a dominant allele, and short is recessive. In the pea plant flower color, the dominant allele is yellow, and the recessive allele is green.

In these cases, the dominant alleles overrule the recessive alleles in the case of the heterozygous genotype. Human organisms’ eye color and hair color can be observed as an example of alleles.


gene alleles

Multiple Alleles

Alleles or allelomorphs are the alternative forms of a gene present in the same location on the homologous chromosomes. Genes have more than two allelic forms, which are referred to as multiple alleles.

Multiple Alleles Examples

Blood Groups in Humans

The A, B, AB, and O blood groups are the types in humans that are determined by three alleles of a gene. For example, gene I, which determines the blood type, is present in three allelic forms. They are IA, IB, and i. The allele IA makes surface antigen A on the red blood cells, the allele IB makes surface antigen B on the red blood cells, and the allele I has 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. For example, if a person has two IA alleles (IAIA) or one IA and one I allele (IAi), the person’s blood group is the ‘A’ group. Likewise, IBIB and IBi give rise to blood group type ‘B.’ Likewise, IAIB gives rise to the AB blood group, and two i alleles (ii) give rise to the ‘O’ blood group.

Blood group



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