In genetics, like any other science, there isspecific terminology designed to clarify key concepts. Back in school, many of us have heard such terms as dominance, recessivity, gene, allele, homozygosity and heterozygosity, but they did not fully understand what was behind them. We will analyze in more detail what is a homozygote, what is it different from a heterozygote and what role does allelic genes play in its formation.
Some general genetics
To answer the question, what is homozygote,remember the experiences of Gregor Mendel. Crossing different in coloring the fruits and the shape of the pea plant, he came to the conclusion that the result of the crossing of the plant inherits somehow genetic information from its "ancestors". Although the concept of "gene" did not exist then, Mendel was able to outline in general outline the mechanism of inheritance of characters. From the laws discovered by Mendel in the middle of the nineteenth century, the following statement, later called the "hypothesis of the purity of gametes," was derived: "In the formation of gametes, only one of the two allelic genes responsible for this feature falls into it." That is, from each of the parents we get only one allelic gene responsible for a certain sign - growth, hair color, eye color, nose shape, skin tone.
Allelic genes can be dominant orrecessive. This brings us very close to determining what a homozygote is. Dominant alleles can mask a recessive so that it does not manifest itself in the phenotype. If in a genotype both genes are recessive or dominant, then this is a homozygous organism.
Types of homozygotes
From all of the above, you can answer the question,what is a homozygote: a cell in which the allelic genes responsible for a particular feature 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 other is not, then it is a heterozygote (Aa). In the case where the genotype of the cell is aa, it is a recessive homozygote if AA is dominant, since it carries alleles responsible for the dominant trait.
Crossing Features
When two identical (recessive or dominant) homozygotes are crossed, a homozygote is also formed.
For example, there are two white flowers of rhododendron with bb genotypes. After their crossing, we also get a white flower with the same genotype.
You can also give an example with the color of the eyes. If both parents have brown eyes and they are homozygous for this symptom, then their genotype is AA. Then all children will have brown eyes.
However, crossing homozygotes does not always lead toformation of homozygous for some sign of the organism. For example, crossing the red (DD) and white (dd) carnations can lead to the formation of a pink or red-white flower. Pink carnation, as well as two-color, is an example of incomplete dominance. In both cases, the resulting plants will be heterozygous with the Dd genotype.
Examples of homozygotes
There are many 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, homozygous organisms, either very light skin, blue eyes, blond hair or color blindness, are often formed.
Dominant homozygotes also occur frequently,However, due to the ability of the dominant signs to mask the recessive, one can not say at once whether a person is a carrier of a recessive allele. Most of the genes responsible for genetic diseases are caused by gene mutations and are recessive, so they only appear if homologous chromosomes do not have a normal, dominant allele.
