Two black mice have a litter that includes brown offspring. Black fur is dominant to brown fur. Which explanation best accounts for the presence of brown offspring?

• A. Both parents are heterozygous for black fur (Bb).
• B. One parent is brown and the other is black (bb x BB).
• C. Both parents are homozygous dominant (BB).
• D. Brown fur is a dominant trait in this cross.

Answer: (A)

The key idea is that a recessive trait can appear in offspring only when both parents carry the recessive allele. Black is dominant, so a mouse looks black if it has BB or Bb, while brown appears only if it is bb. Seeing brown offspring means each parent contributed a recessive b allele, so both parents must be heterozygous Bb. When Bb is crossed with Bb, there is a 1 in 4 chance of bb offspring, which would be brown, matching the observation. If one parent were BB or if brown were the dominant trait, brown offspring would not appear in the litter.