The offspring of two heterozygous gray-bodied, normal-winged flies should be 50% gray-bodied/normal wings (BbRr) and 50% black-bodied/vestigial wings (bbrr) because these alleles are linked. Suppose a small number, say 15%, of the offspring are instead black-bodied with normal wings. This is most likely the result of

A) crossing-over.
B) incomplete dominance.
C) codominance.
D) an error in meiosis.
E) mutation.

Crossing-over during meiosis can result in the exchange of genetic material between homologous chromosomes, leading to recombinant offspring that have different allele combinations than what would be expected if the genes were perfectly linked. In this case, the presence of black-bodied, normal-winged flies among the offspring suggests that crossing-over occurred between the genes controlling body color and wing type, creating new allele combinations.