Practice question

For tightly linked loci double crossover requiring two exchange events plus participation four chromatids to produce nonparental ditype extremely improbable, especially when positive interference suppresses nearby second exchange through chromosome axis signaling. Hence NPD tetrads rarest among three classes, while PD most frequent from no crossover and TT intermediate frequency resulting single crossover. Rarity NPD forms diagnostic criterion linkage strength: as linkage tightens NPD approaches zero counts. As genes become farther apart NPD rises toward PD frequency, reaching equality when unlinked, serving quantitative indicator map distance and interference strength.