An experiment was performed to determine the mode of inheritance of two mouse genes, one for fur color and one for fur length. It is known that black fur (B) is dominant over white fur (b) and that long fur (L) is dominant over short fur (1). To determine how the genes are inherited, a cross was performed between two true-breeding mice, one with long black fur and one with short white fur. Their progeny, the Fi generation, all had long black fur. Five Fı male-female pairs were then crossed with one another. The F2 generation phenotypes for each cross are shown in

The F2 generation phenotypes for each cross are shown in Table 1. (See attachment) Which of the following is the mean number per cross of F2 generation offspring that are the result of crossing over?

Answer:

B, 2.2

Explanation:

The parental genotypes are long and black vs short and white. Therefore the phenotypes that result from crossing over are long and white, and short and black. (middle two rows of the table). If we add up the total number of offspring with these genotypes we get 6 long whites, and 5 short blacks.

The total is 11 from 5 crosses, so the mean is 11/5 = 2.2