In mice, the spinning behavior is caused by a dominant gene that affects the mouse's equilibrium.

Question

This gene is lethal (deadly) if two alleles are present. Two "spinning mice" are mated together. Show the cross. What are the phenotypes of the offspring and in what proportion. (Be sure to eliminate any offspring that do not survive)

Answer

S := spinning
s := not spinning

Two "spinning mice" are mated together. Show the cross. 
As this gene is lethal (deadly) if two alleles are present, the two mice have to be of genotype Ss :

Ss + Ss -> SS, Ss, Ss, ss.

What are the phenotypes of the offspring and in what proportion.

SS (25%) will die off. So 2/3 of the surviving offspring will be spinning mice (genotype Ss), and 1/3 will be non-spinning mice (genotype ss).

In mice, the spinning behavior is caused by a dominant gene that affects the mouse's equilibrium.

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