Homozygous and heterozygous

homozygous individual is one who has two equal alleles of a gene. heterozygous individual is one who has different alleles for a certain characteristic. When we speak of homozygous or heterozygous we refer to a gene for a specific trait. Thus, an individual can be homozygous for one gene and heterozygous for another.

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

At fertilization, a female sex cell or ovum and a male sex cell (sperm) come together and form a zygote with the full genetic makeup of the species. An individual receives a gene from the father and a gene from the mother for the same trait, that is, they have two alleles for the same trait. In humans there are approximately 21,000 different genes.

DefinitionHas two identical alleles of a genePossesses two different alleles of a gene
Origin of allelesFrom the parentsFrom the parents
  • Dominant
  • Recessive
ExamplesThe albino color in the skin of some animals in a homozygous characterPeople with blood type AB are heterozygous

What is homozygous?

When the alleles of a gene are the same, we speak of a homozygous individual. In the case of rabbits, if a rabbit receives both brown alleles it will be homozygous for the brown color, and if it has two albino alleles, it will be homozygous for the albino color.

Dominant homozygous

When two equal alleles that express the dominant phenotype are combined, we are in front of a dominant homozygous. For example, the brown color of the eyes in humans is a dominant character, if the person has both alleles for brown, he will be homozygous dominant.

Recessive homozygous

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, they have two O alleles (OxO).

See also Gene and allele .

What is heterozygous?

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

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

Allele combinationBlood typeCharacter

The combination of alleles would 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.

See also Genotype and phenotype .

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