Understanding Dominant vs. Recessive Alleles

Steps:

• Recall definitions: Dominant alleles express in heterozygotes; recessive alleles express only in homozygotes.
• Evaluate A: Heterozygotes show dominant phenotype, so true.
• Evaluate B: Homozygous can be AA (dominant) or aa (recessive), so true.
• Evaluate C: Recessive phenotype requires aa genotype, so true.
• Evaluate D: Homozygous recessive shows recessive phenotype, so false.

Why D is correct:

• By Mendel's law of dominance, homozygous recessive (aa) expresses the recessive trait, not always dominant.

Why the others are wrong:

• A is correct: Dominant allele masks recessive in heterozygotes (Aa shows dominant phenotype).
• B is correct: Homozygous dominant (AA) or recessive (aa) both possible.
• C is correct: Recessive phenotype only appears with two recessive alleles (aa).

Final answer: D