Unlinking genes by crossing over. Linked genes can become unlinked by chromosome recombination, or crossing over, during meiosis. The chromosomes containing the linked genes are illustrated in diploid cells and in gametes. To analyze gametes produced by F1 flies, the F1 hybrid is crossed with a double-recessive fly of genotype bbvv. In the F1 gametes, B and v are linked, and b and V are linked, so all F2 progeny are expected to contain these same two combinations. The double-recessive fly always contributes a bv gamete. But, in fact, four types of F2 progeny are observed: two are the expected phenotypes; the other two contain b, v and B, V, resulting from gametes in which the linked alleles were unlinked by recombination during meiosis. The two crossover phenotypes are produced at equal frequency (17% each).