One of the most significant properties of any living organism is heredity, which underlies the evolutionary processes on the planet, as well as the conservation of species diversity on it. The smallest unit of heredity is a gene, a structural element of a DNA molecule that is responsible for transmitting hereditary information related to a particular trait of an organism. Depending on the degree of manifestation, dominant and recessive genes are distinguished. A characteristic feature of dominant units is the ability to “suppress” recessive ones, having a decisive effect on the body, not allowing them to manifest themselves in the first generation. However, it is worth noting that, along with complete dominance, an incomplete one is observed in which the dominant gene is not able to completely suppress the manifestation of recessive and superdomination, which involves the manifestation of the corresponding traits in a stronger form than in homozygous organisms. Depending on which allelic (that is, responsible for the development of the same trait) genes it receives from the parental individuals, heterozygous and homozygous organisms are distinguished.
Definition of a homozygous organism
Homozygous organisms are living objects that have two identical (dominant or recessive) genes according to one or another attribute. A distinctive feature of subsequent generations of homozygous individuals is their lack of splitting of characters and their uniformity. This is mainly explained by the fact that the genotype of a homozygous organism contains only one type of gametes, denoted by an uppercase letter in the case of dominant characters, and a lower case when recessive ones are mentioned. Heterozygous organisms are distinguished by the fact that they contain different allelic genes, and, in accordance with this, form two different types of gametes. Homozygous organisms recessive in the main alleles can be designated as aa, bb, aabb, etc. Accordingly, homozygous organisms dominant in alleles have the code AA, BB, AABB.
Patterns of inheritance
The crossing of two heterozygous organisms, the genotypes of which can be arbitrarily designated as Aa (where A is the dominant and a recessive gene), makes it possible to obtain with equal probability four different combinations of gametes (genotype variant) with a 3: 1 splitting by phenotype. Under the genotype in this case refers to the totality of genes that contains a diploid set of a cell; under the phenotype - a system of external as well as internal signs of the organism in question.
Hybrid Crossing and Its Features
Consider the laws associated with the processes of crossing, in which homozygous organisms take part. In the same case, if there is a hybrid or polyhybrid crossing, regardless of the nature of the inherited traits, splitting occurs in a ratio of 3: 1, and this law is true for any number of them. The crossing of individuals of the second generation in this case forms four main types of phenotypes with a ratio of 9: 3: 3: 1. It is worth noting that this law is valid for homologous pairs of chromosomes, the interaction of genes within which is not carried out.
