Black females from two different strains of mice were crossed with brown males. From the first line of mice

Black females from two different strains of mice were crossed with brown males. From the first line of mice, 50% of black and 50% of brown mice were obtained; from the second line – 100% black mice. Explain the results of the experiments

Proceeding from the characteristic splitting among the offspring and the obvious from the condition of the problem of the dominance of the gene of black coat color over the gene of brown, we will introduce symbols and solve the problem for the convenience of explaining the results of the experiments.

Let the gene for the development of black color in mice be B, then the gene for brown color will be b.

Brown mice will be cc, because homozygosity is a prerequisite for the phenotypic realization of a recessive trait.

In order for the offspring to produce brown mice, both parental individuals must pass on the brown gene, that is, b.

Brown males produce the same type of sperm and pass them on to all their offspring.

Black females, in order to give birth to brown mice, must be heterozygous – Bb and produce two types of eggs: B and b.

From crossing such females with brown males, 50% of the offspring will have Bb and have black hair, 50% – bb, and have brown hair, which satisfies the results of the experiment.

Since the crossing of brown males (bb) with another line of black mice did not lead to the appearance of a single brown mouse, it should be concluded that this line is homozygous for the dominant gene – BB. Such black mice produce the same type B eggs and pass them on to all mice, 100% of which have the Bb genotype.