LINKAGE 



with each other and with the relation of the 

 factors in the Fl parent. 



Whether there is linkage and what may be the 

 best estimate of the percentage of crossovers 

 cannot necessarily be determined from the ratio 

 (AB + ab)/(AB + aB + Ab + ab) . Muller (1916) 

 has shown a convenient method, The Square Root 

 of the Product Method, for determining gametic 

 ratio, recombinations (r) to straights (s), 

 which compensates for viability differences. 

 If viability is affected proportionally by tv/o. 

 pairs of alleles, regardless of their combina- 

 tion, and if the numbers of the genotypes be. 

 AB, aB, Ab, and ab respectively, the gametic ra- 

 tio, r/s equals ( 1/aB X ab : 1/aB X Ab) for 

 the repulsion test and ( VaB X Ab : 1/AB X ab) 



for the coupling test. If, however, the factors 

 differ in their effects on viability according 

 to their various combinations with each other 

 and if genotypes from the repulsion cross be 

 ABl, aBl, Abl, abl, and from the coupling cross 

 AB2, aB2, Ab2, and ab2, then gametic ratio should 

 be equal to any one of the four expressions: 



AB2 X Abl y ABg X aB^ y abg X Ab-j^ y ebg X aB^* 



117 



