This is known as a 3 factor cross sh cn sh 900 912 18 22 These are all of the possible combinations. One pair of these gamete classes must be the result of double crossovers. This class will be very rare(0.1 x 0.02 =2 x 10-3).By finding the rare class we have a qualitative test to determine gene order The double crossover classes for the two possible orders are Ch sh (If this were the order these would be the rare classes Cw hpce (Since these are the rare classes we know this to be the order)This is known as a 3 factor cross O cn sh vg x O cn sh vg + + + cn sh vg cn sh vg 900 + + + 912 cn + + 2 +shvg 1 cn sh + 75 + + vg 70 cn + vg 18 +sh+ 22 These are all of the possible combinations. One pair of these gamete classes must be the result of double crossovers. This class will be very rare (0.1 x 0.02 = 2 x 10-3). By finding the rare class we have a qualitative test to determine gene order. The double crossover classes for the two possible orders are: cn + vg or + sh + (If this were the order, these would be the rare classes) sh + vg or + cn + (Since these are the rare classes we know this to be the order)