Difference between homozygous and heterozygous
An individual homozygous It is one that has two equal alleles of a gene. An individual heterozygous It is one that has different alleles for a certain characteristic. When we speak of homozygous or heterozygous we mean a gene for a specific trait. Thus, an individual can be homozygous for one gene and heterozygous for another.
The genes They are fragments of DNA that determine a specific character of the living being. A gene for a characteristic can have several versions, which is 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. There are approximately 21,000 different genes in humans.
| Homozygous | Heterozygous | |
|---|---|---|
| Definition | Has two identical alleles of a gene | Possesses two different alleles of a gene |
| Origin of alleles | From the parents | From the parents |
| Types |
|
|
| Examples | The albino color in the skin of some animals in a homozygous character | People 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 identical alleles expressing 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.
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, 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 allele combinations from their parents:
| Allele combination | 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 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.
Doctor in Biochemistry from the Venezuelan Institute of Scientific Research (IVIC), with a degree in Bioanalysis from the Central University of Venezuela.