Misconception in Homozygous Phenotypes

Steps:

• Recall that dominant alleles mask recessive ones in heterozygotes, determining phenotype.
• Identify homozygous genotypes: two dominant alleles (homozygous dominant) show dominant phenotype; two recessive alleles (homozygous recessive) show recessive phenotype.
• Evaluate statements: A fits both homozygous types; B requires aa for recessive phenotype; C shows dominant in heterozygotes.
• Spot error in D, as homozygous recessive yields recessive phenotype.

Why D is correct:

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

Why the others are wrong:

• A: Homozygous genotypes include both dominant (AA) and recessive (aa) forms.
• B: Recessive phenotypes require two recessive alleles (aa) to express, as one dominant masks it.
• C: Heterozygotes (Aa) display the dominant phenotype due to masking.

Final answer: D