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?


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


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 3:1 ratio of black:brown in F2 is explained if black is dominant.

Reason: BB (black) × bb (brown) → all F1 are Bb (black). Bb × Bb produces genotypes BB:Bb:bb in a 1:2:1 ratio, giving phenotypes 3 black : 1 brown. Your observed 961 black : 317 brown (total 1278) is very close to the expected 958.5 : 319.5, consistent with black being dominant over brown.