Question

A geneticist crossed pure breeding black (BB) mice with pure breeding brown mice (bb). All the 992 mice in the F1 generation had black coats. When these F1 mice were crossed, they yielded 961 black coated mice and 317 brown coated mice. How could you account for the ratio of black coated to brown coated mice in the F2 generation?


The black allele is not independent from the brown allele and disappears in the F1 generation.


An approximately three-to-one (3:1) ratio of black to brown coated mice in F2 is accounted for by the brown allele being dominant over the black allele.


The brown allele is not independent from the black allele and disappears in the F1 generation.


An approximately three-to-one (3:1) ratio of black to brown coated mice in F2 is accounted for by the black allele being dominant over the brown allele.

Answer 1

An approximately three-to-one (3:1) ratio of black to brown coated mice in F2 is accounted for by the black allele being dominant over the brown allele.

Reason: BB × bb → all F1 Bb (black). Bb × Bb (F1 cross) gives genotypes 1 BB : 2 Bb : 1 bb, so phenotypes 3 black : 1 brown. Expected from 1278 offspring is 958.5 black and 319.5 brown; the observed 961 black and 317 brown fit this expectation (close to 3:1).