MENDELIAN SEGREGATION 5 



number of Mendelian units are known, we should still 

 expect to find some of them in groups. 



In 1906 Bateson and Punnett made the discovery 

 of linkage, which they called gametic coupling. They 

 found that when a sweet pea with factors for purple 

 flowers and long pollen grains was crossed to a pea 

 with factors for red flowers and round pollen grains, 

 the two factors that came from the same parent 

 tended to be inherited together. Here was the first 

 case that gave the sort of result that was to be ex- 

 pected if factors were in chromosomes, although this 

 relation was not pointed out at the time. In the 

 same year, however, Lock called attention to the 

 possible relation between the chromosome hypothesis 

 and linkage. 



In other groups a few cases of coupling became 

 known, but nowhere had the evidence been sufficiently 

 ample or sufficiently studied to show how frequently 

 coupling occurs. Since 1910, however, in the fruit 

 fly, Drosophila melanogaster, a large number of new 

 characters have appeared by mutation, and so rap idly 

 does the animal reproduce that in a relatively short 

 tim e the inheritanc e o f more than a hun dred char- 

 a cters has been studied. It became evide nt very soon 

 that these characters are inherited in groups. ThereN 

 is oncT great group of characters^ that. are sex linkedy 

 There are two other groups of characters slightly 

 greater in number. Finally a character appeared 

 that did not belong to any of the other groups, and a 

 year later still another character apj^eared that was 

 linked to the last one but was indei)endent of all the 



