For two unlinked genes, PD and NPD occur in ratio
Two loci unlinked either located on distinct chromosomes or far apart on same chromosome behave independently during metaphase alignment and recombination. Random segregation of chromatids among four products from independent orientation produces equal proportions parental versus nonparental ditype configurations because orientation does not influence allele combinations beyond chance probability. Expected frequencies under independence: PD 25 percent, NPD 25 percent, TT 50 percent from two independent segregation patterns involving different chromatid combinations. Observed ratio PD:NPD near
Ref: Griffiths et al., Introduction to Genetic Analysis, 12th ed., Chapter 5: Unlinked Genes PD Equals NPD 1:1