Tetrad (genetics)

Parent dytype (P) is the type of tetrads that combines in its genotypes the two studied genes in the parental variant (for example, when crossing AB × ab, in this case parental dotypes AB and ab appear). In ascomycetes, splitting in spores corresponds to this type, in which there are only two non-recombinant ascospore types (AB, AB, ab, ab).

The non-parent ditip (N) is such a tetrad that only two recombinant genotypes (aB and Ab) are found in four spores.

In the case of tetratype, 4 different genotypes are found in the tetrad, two of which are recombinant (aB and Ab), and two are not (AB and ab). In marsupials, the formation of tetratype indicates the presence of a single crossing-over between two linked loci .

Before meiosis, the DNA of both sets of chromosomes has zygotes doubled, so double-set chromosomes now contain 2 chromatids (i.e., the zygote has the genetic formula 2n4c , where n is the number of haploid sets of chromosomes and c is the amount of DNA). A nucleus that contains two sets of such chromosomes is divided into two stages, divided into four new nuclei ( 2n4c → n2c → nc ). Each of them has a single (haploid) set of monochromatic chromosomes. After this process, each of the four new nuclei doubles the DNA again and shares mitosis ( n2c ). As a result, an asuka with four pairs of spores is formed.

So, each of the ask (bags) contains 4 haploid spores with twice the amount of DNA, that is, they have the genetic formula n2c . Therefore, the cleavage in the asks (bags) corresponds to the gametic cleavage (after I division of meiosis, when recombination occurs, the progenitors of the germ cells have the genetic formula n2c , which upon subsequent mitotic division becomes nc , since each cell gives one set of DNA to daughter cells) ^[2] .

For this reason, the cleavage in tetrads of the monoheterozygote A / a corresponds to 2A: 2a, and for the diheterozygote AB / ab there are 3 types of tetrads described above: the parent type 2AB: 2av, the non-parent type 2Av: 2aV and the tetratype 1AB: 1Av: 1aB: 1av. The frequency of occurrence of each of these types of tetrads gives the right to draw conclusions about the linkage of genes and centromeres (linked sites recombine together). In addition, based on the fact that the crossover frequency depends on the distance between genes, one can also judge the distance between genes or genes and centromeres ^[3] .

Notebook analysis helped to establish that crossing over occurs at the stage of four rather than two chromatids (although theoretically this option was possible). The marsupial fungus neurospore thick ( Neurospora crassa ) was chosen as the object of research. The peculiarity of the neurospores is that ascospores are linearly located in the ascus , and the direction of chromosome divergence coincides with the long axis of the ascus. Four haploid nuclei after meiosis are again divided by mitosis, as a result, 4 pairs of haploid spores are arranged in the same row in the aska, and the genotype of each pair must be identical.

If crossing over occurred at the stage of two chromatids, then when AB: ab was crossed, a single linear arrangement of Ab-Ab-Ab-aB-aB-aB would be observed. In reality, much more complex variants of arrangements are usually observed, for example, AB-AB-Ab-Ab-aB-aB-ab-ab and Ab-Ab-AB-AB-aB-aB-ab-ab. This is due to which particular chromatids of the four entered into recombination ^[1] .

Technically, such observations are carried out using the technique of micromanipulation, which allows isolating under a microscope each of the four spores of the asuka. After germination under suitable conditions, spores form clones, which allows one to determine their phenotype , and hence the genotype ^[3] .