Silkworm Gene Linkage & Exchange

The phenomenon that two non-alleles are located on the same chromosome and inherited together is called gene linkage. In the process of sex cell meiosis, certain gene-carrying fragments between non-sister chromatids are exchanged, which is called gene exchange, which forms a new gene combination. There are about 320 silkworm chain genes confirmed so far, but the chain and exchange of silkworm genes is different from most organisms. Genes of general organisms are linked in male and female individuals and are basically the same. However, in silkworm gene exchange, it is only seen in males and not in females, which is exactly the opposite of fruit flies and flesh flies, which are only seen in females and not in males. However, from the point of view of the composition of sex chromosomes, both of them are completely consistent in that there is no exchange in heteromatch sex (XY or ZW).

Yoshihide Tanaka (1913) discovered that the markings of silkworms are linked to the blood color. The yellow blood of the common silkworm was crossed with the white blood of the prime silkworm, and the first generation of the yellow blood of the common silkworm was the common silkworm. For example, if this hybrid 1 generation female backcrosses double recessive silkworm white blood male, then it will backcross 1 generation and only give birth to common silkworm yellow blood and silkworm white blood of the same type of parent, but not common silkworm white blood and silkworm yellow. Individuals with blood genotypes. However, in the first generation of male mating silkworm white blood females, in addition to the same type as the parent in the first generation of backcrossing, a small number of common silkworm white blood and silkworm yellow blood were isolated, accounting for approximately the total number of individuals in the first generation of backcrossing. 25% of it. This kind of linkage phenomenon between the dominant gene and the dominant gene is called co-citation; on the contrary, if the common silkworm white blood mating element silkworm yellow blood is used, the first generation is also the common silkworm yellow blood, and the aforementioned hybrid 1 representative type same. However, in this case, the yellow-blooded female backcross of the common silkworm with double recessive silkworm white blood, its backcross 1 generation and the previous backcross 1 generation are completely opposite in phenotype and species, and only common silkworm white blood and silkworm are born. There are two types of yellow blood, which do not give birth to individuals with the genotypes of common silkworm yellow blood and silkworm white blood. However, if one generation of males mate with double recessive white-blood females of silkworms, in addition to the two parental types, a small number of common silkworm yellow blood and silkworm white blood are also isolated, accounting for about the total number of individuals in the first generation of backcrossing. 25%, this kind of linkage phenomenon between dominant and recessive genes is called repulsion. The above two crosses, regardless of mutual attraction or repulsion, although the rate of phenotypic type in the first generation of backcrossing is completely opposite, the gene combination of the same phenotype as the original crossed parents is higher than the gene combination of the new phenotype. Many, on this point, the two are exactly the same. Therefore, the two are called chain. The phenomenon that a small number of individuals with different genotypes from their parents are born after the male backcross of these hybrids is called partial linkage, while the phenomenon that the females of the first hybrid backcross completely do not produce individuals with genotypes different from their parents, which is called complete linkage. .

The study of which chromosome and which linkage group each gene belongs to is not only very important in genetics, but also of great significance for the improvement and production of silkworm breeds. In particular, the genes that determine the length of the silkworm’s elapsed days, disease tolerance, and various quantitative traits, which chromosomes are contained in and which genes are linked to, need to be studied clearly. Because these genes that determine practical traits are linked to genes with silkworm color and specific markings (both on the same chromosome), then when the variety is improved for these practical traits, the cocoon color or color can be easily seen. The markings are used as a symbol for breeding selection. For chemistry, studies have been conducted on the number of days or cocoon traits, and it has been clear that the exchange pattern of any two linked genes in silkworms can only be obtained from the backcross offspring of male silkworms of these two genes, and cannot be obtained from them. Obtained by female silkworms.

X-rays are irradiated in the pupal stage, and sometimes female silkworms can produce very few exchange patterns. Although the exchange rate between the two genes in the male silkworm is the basis for formulating the chromosome map, these exchange rates also vary due to different conditions such as the feeding temperature and chromosome aberrations.