LINKAGE 



Round Yellow Wrinkled Yellow Round Green Wrinkled Green 

 315 loi 108 32 



All but 27 of these plants set seed by selfmg and their constitutions 

 proved to be as shown in Table 3 : 



TABLE 3 



THE NINE GENETICAL CLASSES IN AN F2 SEGREGATING 

 FOR TWO FACTORS (MENDEL) 



Thus the segregation ratio, as we may call it, is i : 2 : i for R-r 

 in each of the Y-y classes and vice versa. The factors are recombined 

 at random, so that the whole table represents the square of a i : 2 : i 

 ratio. 



Applying this square rule, if for one factor there are two kinds 

 of gametes, for two factors there must be four kinds, produced in 

 equal numbers {RY, Ry, rY, and ry). This conclusion Mendel also 

 verified by back-crosses. He even extended it by testing the indepen- 

 dence of three factors, where of course eight kinds of gametes are 

 produced in equal numbers. This principle of independent recom- 

 bination is known as Mendel's second law. 



Linkage 



Since Mendel's work was rediscovered in 1900, the application 

 of these rules has been studied in nearly all groups of sexually repro- 

 ducing organisms. They have been universally verified with one 

 modification, itself also universal : different factors are not always 

 independent of one another in their segregation. 



In the F2 of a cross between two strains of Sweet Pea, Lathyrus 



41 



