In genetics, like any other science, there is a specific terminology designed to clarify key concepts. Even at school, many of us heard such terms as dominance, recessiveness, gene, allele, homozygosity and heterozygosity, but did not fully understand what was behind them. Let us examine in more detail what a homozygote is, how it differs from a heterozygote, and what role allelic genes play in its formation.
A bit of general genetics
To answer the question of what is homozygote, let us recall the experiments of Gregor Mendel. Crossing various pea plants in color and shape, he came to the conclusion that the plants resulting from the crossing inherit in some way the genetic information from their “ancestors”. Although the concept of “gene” did not yet exist, Mendel managed to outline in general terms the mechanism of inheritance of characters. From the laws discovered by Mendel in the middle of the 19th century, the following statement followed, later called the "gamete purity hypothesis": "When a gamete is formed, only one of the two allelic genes responsible for this trait gets into it." That is, from each of the parents we get only one allelic gene that is responsible for a certain trait - growth, hair color, eye color, nose shape, skin tone.
Allelic genes can be dominant or recessive. This brings us very close to determining what a homozygote is. Dominant alleles are able to mask the recessive so that it does not manifest itself in the phenotype. If both genes are recessive or dominant in the genotype, then this is a homozygous organism.
Types of homozygotes
From the foregoing, one can answer the question of what is homozygous: this is a cell in which the allelic genes responsible for a particular trait are the same. Allelic genes are located on homologous chromosomes and, in the case of homozygotes, can be either recessive (aa) or dominant (AA). If one allele is dominant, and the second is not, then this is a heterozygote (Aa). In the case when the cell genotype is aa, then this is a recessive homozygote, if AA is dominant, as it carries alleles responsible for the dominant trait.
Crossbreeding Features
When two identical (recessive or dominant) homozygotes are crossed, a homozygote also forms.
For example, there are two white rhododendron flowers with bb genotypes. After crossing them, we also get a white flower with the same genotype.
You can also give an example with eye color. If both parents have brown eyes and they are homozygous for this feature, then their genotype is AA. Then all the children will have brown eyes.
However, crossing homozygotes does not always lead to the formation of an organism that is homozygous for any characteristic. For example, crossing red (DD) and white (dd) carnations can lead to the formation of a pink or red-white flower. Pink carnation, as well as two-tone, is an example of incomplete dominance. In both cases, the resulting plants will be heterozygous with the Dd genotype.
Examples of homozygotes
There are quite a few examples of homozygotes in nature. White tulips, carnations, rhododendrons are all examples of recessive homozygotes.
In humans, as a result of the interaction of allelic genes, organisms homozygous for some reason, too, often form, be it very fair skin, blue eyes, blond hair, or color blindness.
Dominant homozygotes are also common, but because of the ability of dominant traits to mask recessive ones, one cannot immediately say whether a person is a carrier of a recessive allele or not. Most genes responsible for genetic diseases are caused by gene mutations and are recessive, therefore they appear only if there is no normal, dominant allele on the homologous chromosomes.