Determining the Distances Between Genes in a 3point crosses

Determining the Distances Between Genes in a 3-point crosses Cross: of Individuals offspring Phenotype 3501 3471 28 32 Total 7227

Solution

Solving a Three-Point Cross:-

Draw a map of these 3 genes (W, X and Y) showing the distances between all pairs of genes, and then calculate the value of interference.

NCO(no crossover)

+++

3501

NCO(no crossover)

WXY

3471

DCO(double crossover)

+X+

6

DCO(double crossover)

W+Y

3

SCO(single crossover)

+XY

32

SCO(single crossover)

W++

28

SCO(single crossover)

++Y

98

SCO(single crossover)

WX+

88

                                                                                             Total = 7227

In order for this to be a double crossover, the X allele must be in the center.

Therefore,

• To determine the distance, add the number of times crossover occurred between the two loci. SCO+DCO/ Total Offspring * 100=

W – X distance

DCO(6+3) + SCO(88+98)/7227*100 = 195/7227*100 = 2.70mu

Similarly,

X-Y

DCO(3+6) + SCO(28+32)/7227*100 = 69/7227*100 = 0.954mu

W<-------------X----------->Y

2.7mu                         0.95mu

NCO(no crossover)	+++	3501
NCO(no crossover)	WXY	3471
DCO(double crossover)	+X+	6
DCO(double crossover)	W+Y	3
SCO(single crossover)	+XY	32
SCO(single crossover)	W++	28
SCO(single crossover)	++Y	98
SCO(single crossover)	WX+	88