Non-parental ditype (NPD) tetrad contains
Nonparental ditype contains only recombinant genotypes reciprocal to parents. From parents AB and ab, NPD ascus contains two Ab spores plus two aB spores, each recombinant present twice, no parental types observed. Formation requires four-strand double crossover between markers where all chromatids become recombinant, or independent assortment equivalent creating recombination in all strands. Because need two exchanges involving all four chromatids, frequency extremely low for linked loci but equals PD for unlinked loci. High NPD relative PD contradicts linkage and indicates loci on different
Ref: Hartl & Ruvolo, Genetics, 6th ed., Chapter 5: NPD Contains Only Recombinant Spores Mechanism
