Homologous chromosomes carry the same genes at corresponding loci

Steps:

• Identify P and Q as corresponding positions on a pair of homologous chromosomes in the same cell.
• Recognize that a heterozygous individual has two different versions (alleles) of the gene at that locus.
• Note that homologous chromosomes contain the same genes, but alleles may differ.
• Conclude that Q holds the alternative allele for the identical gene found at P.

Why A is correct:

• Alleles are variant forms of the same gene, positioned at equivalent loci on homologous chromosomes, as defined in Mendelian genetics.

Why the others are wrong:

• B: Homologous chromosomes have the same gene at corresponding positions, not different genes.
• C: Alleles apply to the same gene; different genes cannot share an allele.
• D: Identical alleles at both positions indicate homozygosity, not heterozygosity.

Final answer: A