Practice question

Test cross to homozygous recessive tester reveals gamete types produced by heterozygous parent because tester contributes only recessive alleles so progeny phenotype directly reflects gamete genotype from heterozygote. Parental non-recombinant chromosomes retaining original allele configurations occur most frequently when linkage present, while crossover products produce minority recombinant classes. Identifying most frequent two classes as parental enables gene order deduction in three-point crosses where double crossovers rarest. Principle underlies linkage map construction without direct gamete genotyping and simplifies genetic distance calculations reliably.