Fur color inheritance via simple dominance

Steps:

• White fur is recessive (bb genotype); black is dominant (BB or Bb).
• The white mouse from the litter is bb.
• It mates with a heterozygous female (Bb).
• Punnett square: Bb × bb yields ½ Bb (black) and ½ bb (white).

Why A is correct:

• Monohybrid cross of heterozygote (Bb) and recessive homozygote (bb) produces 1:1 black:white ratio, per Mendel's law of segregation.

Why the others are wrong:

• B: Introduces grey phenotype, implying incomplete dominance or codominance, unsupported by data.
• C: Suggests three phenotypes, requiring multiple alleles or epistasis, not present.
• D: Matches heterozygote × heterozygote cross (3:1), but mating is with recessive homozygote.

Final answer: A