Practice question

Three-point cross incorporates three heterozygous markers simultaneously, generating eight phenotypic classes distinguishable as parental nonrecombinant, two single crossover classes for each interval, and one double crossover class rarest. Double crossover class reveals linear order because alleles of middle gene are swapped relative to parental chromosomes, while outer markers remain parental. Recombination frequencies calculated between adjacent pairs separately yield interval distances; sum provides outer interval distance more accurate than direct two-point because double crossovers counted. Coefficient coincidence and interference also derivable.