346 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 3 5 



genetic theory, especially in relation to the discovery of additional 

 laws, because the recognition of a mechanism that can be seen and 

 followed demands that any extension of Mendel's theories must con- 

 form to such a recognized mechanism, and also because the apparent 

 exceptions to Mendel's laws that came to light before long might in 

 the absence of a known mechanism have called forth purely fictitious 

 modifications of Mendel's laws or even seemed to invalidate their 

 generality. We now know that some of these " exceptions " are due 

 to newly discovered and demonstrable properties of the chromosome 

 mechanism and others to recognizable irregularities in the machine. 



Mendel knew of no processes taking place in the formation of 

 pollen and egg cell that could furnish a basis for his primary assump- 

 tion 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 element; 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 35 years 

 between Mendel's paper in 1865 and 1900. It is here that the names 

 of the most prominent European cytologists stand out as the dis- 

 coverers of the role of the chromosomes 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 inheritance. 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 characters 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 chromosome 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 



