Practice question

Double crossover requires two separate exchange events within short region between three genes. Probability product of two single crossover probabilities makes event relatively uncommon, further suppressed by positive interference that inhibits nearby second exchanges via chromosome axis signaling. Consequently progeny formed by double recombination represent smallest numeric class among testcross offspring, whereas parental nonrecombinant class most frequent and single crossovers intermediate frequency. Recognizing least frequent class as double crossover is key step in mapping because it identifies middle gene and allows interference measurement.