134 PHYSICAL BASIS OF HEEEDITY 



In Drosophila melanogaster the evidence is now very 

 strong in favor of the identity in number of linkage groups 

 and chromosome-pairs. As the new characters coming up, 

 one after the other, have continued to fall into the four 

 kno^\Ti groups, and as something like 200 characters have 

 been so placed, and as none of them has failed to show link- 

 age with one of the four established series, the probability 

 is enormously in favor of a causal relation between the two 

 events, especially in the light of the evidence from other 

 sources that the chromosomes are the bearers of the 

 hereditary factors — evidence from the sex-chromosomes, 

 for example. 



The only other species in which the heredity of known 

 mutant characters approaches that of the chromosome 

 group is the garden pea in which about 35 mutant factors 

 have been studied. From the summary of what has 

 been so far recorded, as well as from the results of his 

 own work. White has recently given an account of what 

 is fairly well established. Of the 35 mutant factors 

 in this pea, seven independently inherited groups have 

 been recorded, i.e., each one of seven factors has been 

 tested out and found to assort independently of the other 

 six. There are seven pairs of chromosomes in the edible 

 pea (Fig. 53, a). The agreement between the two is to 

 date perfect. It is, of course, possible that the linkage 

 between some of the factors tested was so loose that they 

 a])peared to give free assortment, and that until more fac- 

 tors have been studied the evidence is not above sus- 

 picion. Nevertheless, it is important to find that the 

 number of independent mutant factors in Pisum sativum 

 does not exceed the number of chromosome pairs. 



White's study of the linkage of factors in the edible 

 peas shows further that there are four linkage groups-— 

 three of them include factors that are also included in 

 those that freely assort. It is fair, perhaps, to conclude 

 that four of the possible seven-linked groups have been 

 found. There are no other forms known in which the 



