236 PHYSICAL BASIS OF HEREDITY 



pairs of contrasted characters are involved in the same 

 cross, we get further information as to the situation. 



For example, Mendel showed that when peas that are 

 both yellow and round are crossed to peas that are both 

 green and wrinkled, there appear in the F2 generation not 

 only the original combinations, but also recombinations of 

 these, viz., yellow and wrinkled; and green and round 

 (Fig. 106). Here also the numerical results 9:3:3:1 

 can be explained on the theory that the representatives 

 of each pair of characters separate in the germ-plasm, 

 and that the separation of each pair is independent of 

 what takes place in the other pair. Obviously it can no 

 longer be whole germ-plasms that separate, but there 

 must be different pairs of elements in the germ-plasm that 

 assort independently of each other. It has been found that 

 this principle of independent assortment may apply to a 

 considerable number of pairs of characters segregating at 

 the same time. The only restriction that is found is in 

 the case of linked pairs of characters. This relation will 

 be considered later. 



The independent assortment of the pairs of characters 

 proves that the elements that stand for the characters in 

 the two original germ-plasms may separate from each 

 other. If each such pair of characters represented one 

 of the pairs of homologous chromosomes, the evidence, so 

 far considered, would be in accord with the view that the 

 chromosomes were the ultimate units involved in the proc- 

 esses of segregation and assortment. The chromosomes 

 are, as has been shown, independent units in the germ- 

 plasm. But as Drosophila shows, there are many more 

 pairs of characters than there are pairs of chromosomes. 

 It is obvious that if the chromosomes are the ulti- 

 mate units involved, and remain intact, there could be no 

 more independent pairs of characters than there are pairs 

 of chromosomes. In animals and in plants there are 

 no cases known where there are more independent pairs 

 than there are chromosomes, so that, as has been pointed 



