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 rapidly 
does the animal reproduce that in a relatively short 
time the inheritance of more than a hundred char- 
acters has been studied. It became evident very soon 
that these characters are inherited in groups. There 
is one great group of characters that are sex linked. 
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 appeared that was 
linked to the last one but was independent of all the 
