Significant Interference

A significant negative interference means that the observed relative frequency of double crossovers is more than expected. A significant positive interference means that the observed relative frequency of double crossovers is less than expected.

			Recombinants
Exchange	Chromosomes	Gametes	Number	%
none		AB	0/4	0
		AB
		ab
		ab
two-strand single		AB	2/4	50
		ab
		Ab
		aB
two-strand double		AB	0/4	0
		AB
		ab
		ab
three-strand double		AB	2/4	50
		Ab
		ab
		aB
three-strand double		ab	2/4	50
		aB
		Ab
		AB
four-strand double		Ab	4/4	100
		Ab
		aB
		aB

When a crossover event occurs the average recombination is 50%. In other words, the average of all double-crossover events, is one-half parental gametes and one-half recombinant gametes.

Single two-strand crossover 50% recombinants
Double crossover mean = 1/4(0/4 + 2/4 + 2/4 + 4/4) = 8/16
                                   = 50% recombinants