Difference Between Homozygous and Heterozygous

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Distinguish Between Homozygous and Heterozygous

Human beings possess two sets of chromosomes - Heterozygous and Homozygous. The terms Homozygous and Heterozygous are used to describe allele pairs. A Homozygous individual carries a set of two identical (RR or rr) alleles while a Heterozygous individual bears different alleles (Rr).

For example, let us consider the gene of the hair colour possessing two alleles, one allele code is responsible black (r) while the other allele code is responsible for black (R). Homozygous individuals will have the same colour allele on a homologous pair of chromosomes, either RR or rr, and Heterozygous individuals will hold different alleles on a homologous pair of chromosomes like Rr.

Let us discuss what these pairs are and the difference between them.

What are Homozygous and Heterozygous Pair of Chromosomes?


Human beings possess the same gene. However, the number of genes are varied, and this controls our physical health and traits. Each variation is called an allele, and every individual inherits two alleles for each gene - one from their biological father and another from their birth mother.

The term ‘Homozygous’ is an amalgamation of two words -  Homo which refers to ’same‘, and zygous, which refers to ‘having zygotes of a specified kind‘.

If the alleles of a gene are identical, then the individual is homozygous for that specific gene. For example, you could possess two identical or similar alleles for a particular gene that causes hazel brown eyes. 

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Some alleles are dominant that is expressed more strongly while some alleles are recessive which get masked by the dominant alleles. However, in a homozygous genotype, the interaction between a dominant and a recessive allele does not occur as the individual holds either two dominant alleles known as homozygous dominant(RR) ot two recessive alleles known as homozygous recessive(rr). 


We know that each gene inherits two alleles and the only difference is the versions of the alleles. The term ‘Heterozygous’ is an amalgamation of two words - Hetero which refer to ‘different’ and zygous which refers to ‘having zygotes of a particular specified kind‘. This results in two different alleles present on the homologous chromosomes.

An individual who possesses a Heterozygous genotype will have two versions of alleles (Rr).  For example, in terms of hair colour, a Heterozygous individual can possess one allele for brown hair and one allele for red hair. The relationship between the two alleles affects the traits expressed in an individual which also determines the characteristics which a carrier possesses. 

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Homozygous vs Heterozygous

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If a diploid organism carries two same or identical copies of the same allele of each gene, where the alleles may be coding for a particular trait, and this is called the homozygous genotype.

If a diploid organism carries two different copies of alleles of each gene, either dominant or regressive, where the alleles may be coding for a particular trait, and this is called the heterozygous genotype.

Homozygous genotype carries a similar but, only one type of allele- either dominant or recessive for a particular trait.

Example- either RR or rr

Heterozygous genotype carries different or dissimilar alleles for a specific trait. It comprises both dominant and recessive alleles.

Example- Rr

A Homozygous individual can either be homozygous dominant or homozygous recessive.

The Heterozygous alleles in an individual can show either complete dominance and co-dominance or incomplete dominance behaviour.

Self-fertilisation in a homologous allele results in the repetition of the identical or similar traits over generations

Self-fertilisation in a heterozygous allele results in different combination of traits over the next generations

For a homozygous allele, only one type of gamete is produced 

For a heterozygous allele, two types of gamete are produced 

Cells or organisms with a homozygous condition fall under the term known as homozygotes.

Cells or organisms with a heterozygous condition fall under the term known as heterozygotes

Homozygous genotypes are observed in organisms that reproduce by asexual means.

Heterozygous genotypes are observed in animals that reproduce by sexual means.

FAQ (Frequently Asked Questions)

Q1. Differentiate Between Homozygous and Heterozygous Pairs of Alleles.

Ans: Suppose a diploid organism carries two duplicates of each gene which may comprise identical alleles. In that case, it is known as homozygous, whereas if a diploid organism has two copies of each gene, which can be different - either as dominant or regressive alleles, it is known as heterozygous.

Homozygous carry a similar set of alleles for a trait. In contrast, heterozygous have different sets of alleles for a feature.

Homozygous results of identical individuals meaning that it is pure for a trait and breeds true. Example, RR or rr while a heterozygous individual results in dissimilar individuals who are rarely pure and produce offspring with a different genotype. Example, Rr.

A homozygous individual can carry either a dominant or a recessive allele while a heterozygous individual possesses both dominant and recessive alleles.

A Homozygous individual produces one type of gamete while a heterozygous individual makes two kinds of gametes.

Q2. State the Different Genes and Disease Caused by a Homozygous Allele.

Ans: A few common genetic conditions that affect homozygous people are- 

Cystic Fibrosis:- Here, you inherit two mutated copies of genesis where the CFTR (Transmembrane Conductance Regulator) gene makes a protein that controls the fluid movement in and out of cells.

Sickle Cell Anaemia:- Here, you inherit two copies of an HBB (Hemoglobin Subunit Beta) mutation which results in the production of mutated abnormal beta-globin that causes inadequate blood supply and low red blood cells through the body.

Phenylketonuria(PKU):- Here, the homozygous individual comprises a PAH (Phenylalanine Hydroxylase) gene mutation where the gene instructs the cells to produce an enzyme that breaks down the amino acid called phenylalanine which accumulates in the tissues and blood.

Methylenetetrahydrofolate Reductase Gene Mutation:- Here, the gene instructs the body to produce the enzyme methylenetetrahydrofolate reductase to break down the homocysteine, however, the gene does not make the enzyme, but results in two mutations.