Difference Between Homozygous and Heterozygous

It is the one that has two equal alleles of a gene, a heterozygous individual is one that has different alleles for a certain characteristic. When we talk about homozygous or heterozygous we refer to a gene for a specific character. Thus, an individual can be homozygous for one gene and heterozygous for another.

Genes are DNA fragments that determine a specific character of the living being. A gene for a trait can have several versions, known as alleles. For example, the gene for rabbit coat color (gene C) has four different alleles: the brown allele, the albino allele, the chinchilla allele, and the Himalayan allele. However, a rabbit can only have two alleles.

In fertilization, a female sexual cell or ovum and a male sexual cell (sperm) come together and form a zygote with the total genetic load of the species. An individual receives one gene from the father and one gene from the mother for the same character, that is, it has two alleles for the same trait. In humans there are approximately 21 thousand different genes.


It is a homozygous individual, in the case of rabbits, if a rabbit receives both brown alleles it will be homozygous for brown, and if it has two albino alleles it will be homozygous for albino.

homozygous dominant

When two identical alleles that express the dominant phenotype are combined, we are dealing with a homozygous dominant. For example, brown eye color in humans is a dominant trait; if a person has both alleles for brown, they will be homozygous dominant.

homozygous recessive

There are certain combinations of alleles that can only be expressed when they are homozygous. We find this in recessive alleles, versions of genes that are not expressed when there is another dominant gene. For example, in the ABO blood group in humans, the O allele is recessive. This means that people with blood type O are homozygous, having two O alleles (OxO).


When both copies of the gene that are inherited from the parents are unequal, the individual is said to be heterozygous for that gene. Continuing with the rabbit coat color example, when a rabbit has one brown allele and one albino allele, the rabbit will be heterozygous for the color brown.

The ABO blood group type in red blood cells is also a good example to explain when a character is homozygous or heterozygous. The gene for blood group has three alleles: A, B, and O. A person can inherit the following combinations of alleles from their parents.

Combination of alleles Blood type Character
AxA TO homozygous
AxO TO heterozygous
BxB B. homozygous
BxO B. heterozygous
AxB AB heterozygous
OxO OR homozygous

The combination of alleles could be the genotype and the blood group is the phenotype. Whenever we see that the alleles are the same (AxA, BxB, OxO) they are homozygous, and when they are different (AxO, BxO, AxB) they are heterozygous.

Homozygous and Heterozygous


homozygous heterozygous
Definition Has two of the same alleles of a gene Has two different alleles of a gene
Origin of the alleles derived from parents It comes from the parents
  • Dominant
  • Recessive


examples The albino color in the skin of some animals in a homozygous character People with blood type AB are heterozygous.

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