Analyzing crossover

Analyzing crossing is a hybrid of a hybrid individual with an analyzer. As the analyzer, a homozygous specimen is chosen, in which the recessive trait is reflected in the phenotype. Analyzing crossover is widely used in breeding and genetics to determine genotype.

As is known, according to the laws of genetics, the recessive trait in mono-hybrid crosses does not appear in the first generation. In the second generation, it manifests itself only in one sixteenth of the individuals. Recessive trait persists in the genotype of the individuals of the first generation, but does not appear externally. Thus, the individual of the first generation, which has a recessive sign in the genotype, does not differ in phenotype from a mono-hybrid individual by a dominant feature. The analyzing crossing is aimed at obtaining information about the presence or absence of a recessive trait in the genotype.

Consider the mechanism for determining the genotype of the specimen of the gray fly. Gray color is the dominant feature in relation to the black color of the body of an insect. The biologist crosses the gray fly with the black and estimates the resulting offspring. If all individuals are gray, the fly studied had only a dominant trait in the genotype. If half of the resulting flies have a black color, then we can conclude that there is a recessive trait.

With incomplete dominance, each genotype has its own phenotype. In order to find out whether any feature observed in an individual is the result of incomplete dominance or an independent sign, genetically laid, it is also necessary to carry out an analyzing crossing. If the test is the result of incomplete dominance, then its "confusion" with the recessive sign of the analyzer will occur. If the sign was independent, it will be transmitted unchanged to all individuals, if the specimen is homozygous, or part of the specimen, if it is heterozygous. For example, if the pink and white flowers were crossed pink-pink, then there is incomplete dominance. If all hybrids of the first order have pink flowers, then this feature is incorporated into the allele as an independent one and dominates over the white one. In this case, the individual is homozygous. If some of the hybrids inherited the pink color of the petals, and the other part was transferred white, then the pink sign is independent, the dominant, the specimen being examined has both characteristics in the genotype.

In practice, to determine the genotype, it is not always sufficient to perform an analysis of crosses. An example falling into the category of exceptions can be explained by one of the three ways of non-allelic interaction of genes: epistasis, polymerase, or complementarity.

When epistasis, the manifestation of the genes of one allelic pair is externally suppressed by the genes of another allelic pair. Suppressors can carry both a dominant and a recessive sign. As a result of the epistasis, when crossing a homozygous individual with red flowers (a dominant trait) and a homozygous individual with white flowers (a recessive trait), only the plants with red flowers will be in the first generation of hybrids , and in the second, 3/4 hybrids will have red color, 3 / 16 is white, and 1/16 will inherit the sign of another allele (for example, yellow).

To explain the effect of the phenomenon of a polymer, we can give the following example. Flowers contain several heterozygous alleles bearing the signs of red (dominant) and white (recessive) petals. The larger the hybrid of the second generation of recessive individuals, the whiter the flower. In the presence of an allele in the individual both with a dominant trait, and without it, externally there is something in between these signs. When determining a genotype through an analysis of crosses, the polymer is easily confused with incomplete dominance.

With complementarity, non-allelic genes complement each other and contribute to the formation of a new trait.