What is the essence of analyzing crosses? Why is this crossing carried out with the obligatory participation of an individual

What is the essence of analyzing crosses? Why is this crossing carried out with the obligatory participation of an individual homozygous for a recessive trait? Illustrate your answer with examples.

Analyzing mating is the mating of an individual with a dominant trait, but with an unknown genotype, with an individual homozygous for a recessive trait, the genotype of which is always aa. According to the result of crossing, the genotype of an individual with a dominant trait is determined. Analyzing mating is often used in breeding plants and animals to determine the genotype of an individual with a dominant trait and to draw a clean line.
Consider the mechanism for determining the genotype of an individual gray fly. Gray color is the dominant feature in relation to the black color of the body of the insect. A biologist crosses a gray fly with a black one and evaluates the resulting offspring. If all individuals have a gray color, then the studied fly had only a dominant trait in the genotype. If half of the resulting flies has a black color, then we can conclude that there is a recessive trait.

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