Dominant black allele from homozygous male produces uniform offspring

Steps:

• Light brown females are homozygous recessive (bb) for color.
• All 20 offspring are black, so each inherits a dominant black allele (B) from the male.
• A heterozygous male (Bb) would produce ~50% brown (bb) offspring, but none appear, indicating male is homozygous (BB).
• Black phenotype in heterozygotes (Bb) confirms black is dominant over brown.

Why D is correct:

• Mendel's law of dominance: homozygous dominant (BB) × recessive (bb) yields all heterozygous (Bb) offspring showing dominant black trait.

Why the others are wrong:

• A: Heterozygous male with brown dominant would yield mostly brown or mixed offspring, not all black.
• B: Heterozygous male (Bb) with black dominant yields ~50% brown (bb) offspring, contradicting uniform black results.
• C: Codominance in homozygous male would produce blended or both-color phenotypes in offspring, not pure black.

Final answer: D