GENETICS, PHYSIOLOGY AND MEDICINE 



cell that could furnish a basis for his 

 primary assumption that the hereditary 

 elements separate in the germ-cells in 

 such a way that each ripe germ-cell comes 

 to contain only one of each kind of ele- 

 ment : but he justified the validity of this 

 assumption by putting it to a crucial test. 

 His analysis was a wonderful feat of 

 reasoning. He verified his reasoning by 

 the recognized experimental procedure 

 of science. 



As a matter of fact it would not have 

 been possible in Mendel's time to give 

 an objective demonstration of the basic 

 mechanism involved in the separation of 

 the hereditary elements in the germ-cells. 

 The preparation for this demonstration 

 took all the thirty-five years between 

 Mendel's paper in 1865 and 1900. It is 

 here that the names of the most promi- 

 nent European cytologists stand out as 

 the discoverers of the role of the chromo- 

 somes in the maturation of the germ-cells. 

 It is largely a result of their work that 

 it was possible in 1902 to relate the well- 

 known cytological evidence to Mendel's 

 laws. So much in retrospect. 



The most significant additions that 

 have been made to Mendel's two laws 

 may be called linkage and crossing over. 

 In 1906 Bateson and Punnett reported a 

 two-factor case in sweet peas that did not 

 give the expected ratio for two pairs of 

 characters entering the cross at the same 

 time. 



By 1911 two genes had been found in 

 Drosophila that gave sex-linked inheri- 

 tance. It had earlier been shown that 

 such genes lie in the X-chromosomes. 

 Ratios were found in the second genera- 

 tion that did not conform to Mendel's 

 second law when these two pairs of char- 

 acters are present, and the suggestion 

 was made that the ratios in such cases 

 could be explained on the basis of inter- 

 change between the two X-chromosomes 

 in the female. It was also pointed out 

 that the further apart the genes for such 

 characters happen to lie in the chromo- 



some, the greater the chance for inter- 

 change to take place. This would give 

 the approximate location of the genes 

 with respect to other genes. By further 

 extension and clarification of this idea it 

 became possible, as more evidence ac- 

 cumulated, to demonstrate that the genes 

 lie in a single line in each chromosome. 



Two years previously (1909) a Bel- 

 gian investigator, Janssens, had de- 

 scribed a phenomenon in the conjugating 

 chromosomes of a salamander, Batraco- 

 seps, which he interpreted to mean that 

 interchanges take place between homol- 

 ogous chromosomes. This he called 

 chiasmatypie — a phenomenon that has 

 occupied the attention of cytologists 

 down to the present day. Janssens' ob- 

 servations were destined shortly to sup- 

 ply an objective support to the demon- 

 stration of genetic interchange between 

 linked genes carried in the sex chromo- 

 somes of the female Drosophila. 



To-day we arrange the genes in a chart 

 or map, Fig. 1. The numbers attached 

 express the distance of each gene from 

 some arbitrary point taken as zero. 

 These numbers make it possible to fore- 

 tell how any new r character that may 

 appear will be inherited with respect to 

 all other characters, as soon as its cross- 

 ing over value with respect to any other 

 two characters is determined. This abil- 

 ity to predict would in itself justify the 

 construction of such maps, even if there 

 were no other facts concerning the loca- 

 tion of the genes ; but there is to-day 

 direct evidence in support of the view 

 that genes lie in a serial order in the 

 chromosomes. 



What are the Genes? 



What is the nature of the elements of 

 heredity that Mendel postulated as 

 purely theoretical units? What are 

 genes? Now that we locate them in the 

 chromosomes are we justified in regard- 

 ing them as material units ; as chemical 

 bodies of a higher order than molecules? 



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