Homologous Chromosomes in Heterozygotes

Steps:

• Homologous chromosomes carry the same genes at corresponding loci.
• Point P marks a gene locus on one chromosome.
• Equivalent position Q is the matching locus on the homologous chromosome.
• In a heterozygous individual, the alleles at these loci differ for that gene.

Why A is correct:

• Heterozygosity means two different alleles of the same gene at the paired loci on homologous chromosomes.

Why the others are wrong:

• B: Equivalent positions indicate the same gene locus, not a different gene.
• C: Alleles are variants of a gene; a different gene cannot share the same allele.
• D: This describes homozygosity, not heterozygosity.

Final answer: A