Two black female mice crossed with a brown male. One female in several litters gave 20 black and 17

Two black female mice crossed with a brown male. One female in several litters gave 20 black and 17 brown, and the second – 33 black. Determine the genotypes of parents and offspring.

According to the results of the second crossing, it can be concluded that the gene that causes the black color of the mice’s coat is dominant. Let’s designate it as C. Then the gene that determines the development of brown wool in these animals will be designated as S.

The brown male will have the cc genotype homozygous for the recessive gene and produce the same type of spermatozoa – c.

In the second crossing, all offspring are uniform and are represented by black mice. From the male, they could only inherit the recessive gene c, so all mice are heterozygotes – Cc. They receive the dominant gene for black wool from a black female, which, given the absence of splitting for the considered trait in the offspring, is homozygous for the gene for black wool – CC. It only produces type C eggs.

In the first cross, a split in color is observed in a ratio approximately corresponding to a 1: 1 ratio. This splitting is characteristic of the so-called analyzing crossing – a heterozygous individual is crossed with a homozygote for a recessive gene. Since the male has brown hair and is homozygous for the recessive gene (cc, gametes – c), the black female in the first crossing is heterozygous. It produces C and C eggs. The offspring are presented in two variants: 50% – heterozygous mice with black wool (Cc) and 50% – brown mice homozygous for the recessive gene (cc).