In meiotic genetics, crossing over between homologous chromosomes can occur more than once along the same bivalent, and double crossovers are classified by how many of the four chromatids in the tetrad participate: two-strand, three-strand, or four-strand double crossovers. Because a two-strand double crossover restores the parental chromatid configuration, the number of observed recombinant products understates the true number of exchange events — the foundation for why genetic map distances derived from recombination frequency underestimate physical exchange over long intervals. The concept belongs to classical and cytogenetic genetics (meiosis, recombination, and linkage mapping), and it extends to the sex chromosomes, where recombination is confined to the pseudoautosomal regions and excluded from the male-specific region to protect sex-determining loci from being transferred between the X and Y.
In meiotic genetics, crossing over between homologous chromosomes can occur more than once along the same bivalent, and double crossovers are classified by how many of the four chromatids in the tetr…