4 MENDELIAN SEGREGATION 



by means of the manoeuvers of the chromosomes 

 seems to have occurred to more than one person, 

 but W. Sutton (1902) first presented the idea in 

 the form in which we recognize it today. More- 

 over, he not only called attention to the fact, above 

 mentioned, that both chromosomes and hereditary 

 factors undergo segregation, but showed that if 

 the pairs assort independently, Mendel's second law 

 ("assortment") is fulfilled. Mendel had found that 

 when the inheritance of more than one pair of factors 

 is followed, the different pairs of factors sort out 

 independently of one another. Thus in a cross of a 

 pea having both green seeds and tall stature with a 

 pea having yellow seeds and short stature, the fact 

 that a germ cell receives a particular member of one 

 pair (e.g., yellow) does not determine which member 

 of the other pair it receives; it is as likely to receive 

 the tall as the short. Sutton pointed out that in the 

 same way the segregation of one pair of chromosomes 

 is probably independent of the segregation of the 

 other pairs. 



It w^as obvious from the beginning, however, that 

 there was one essential requirement of the chromo- 

 some view, namely, that all the factors carried by 

 the same chromosome should tend to remain together. 

 Therefore, since the number of inheritable characters 

 may be large in comparison with the number of pairs 

 of chromosomes, we should expect actually to find 

 not only the independent behavior of pairs, but also 

 cases in which characters are linked together in groups 

 in their inheritance. Even in species where a limited 



