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 the gene - a structural element of the DNA molecule responsible for the transmission of hereditary information associated with a particular trait of the 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, preventing them from appearing in the first generation. However, it should be noted that along with complete dominance, incomplete dominance is observed, in which the dominant gene is not able to completely suppress the manifestation of recessive and overdominance, which provides for the manifestation of the corresponding signs in a form stronger than in homozygous organisms. Depending on which allelic (that is, responsible for the development of the same trait) genes it receives from parent individuals, heterozygous and homozygous organisms are distinguished.
Definitionhomozygous organism
Homozygous organisms are objects of wildlife that have two identical (dominant or recessive) genes for one or another trait. 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 a capital letter when it comes to dominant traits, and a lowercase letter when mentioning recessive ones. 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 that are recessive for the main alleles can be designated as aa, bb, aabb, etc. Accordingly, homozygous organisms that are dominant in alleles have the code AA, BB, AABB.
Patterns of inheritance
Crossing two heterozygous organisms, whose genotypes can be conditionally designated as Aa (where A is a dominant and a is a recessive gene), makes it possible to obtain with equal probability four different combinations of gametes (genotype variant) with 3:1 splitting by phenotype. In this case, the genotype is understood as the totality of genes that contain the diploid set of a particular cell; under the phenotype - a system of external as well as internal features of the organism in question.
Dihybrid crossing and its features
Let's consider patterns related towith crossbreeding processes in which homozygous organisms take part. In the same case, if there is a dihybrid or polyhybrid crossing, regardless of the nature of the inherited traits, splitting occurs in a ratio of 3: 1, and this law is valid for any number of them. Crossing individuals of the second generation in this case forms four main types of phenotypes at a ratio of 9:3:3:1. It should be noted that this law is valid for homologous pairs of chromosomes, the interaction of genes within which is not carried out.
