LINKAGE RELATIONS IN MENDELISM 125 
originally deducted by Muller and Sturtevant from a consideration of 
linkage as a chromosome hypothesis. 
Linkage Phenomena in Other Plants and Animals.—Our extended 
discussion of linkage relations has been based practically entirely on 
Drosophila ampelophila because the factor analysis in this species pro- 
vides us with a body of data incomparably superior to that provided 
by any other species. Nevertheless there are other scattered cases of 
linkage in many species of plants and animals. 
In plants Bateson first described the phenomenon of linkage in sweet 
peas for the characters round pollen and red flower color. Later the 
factor for hooded standard was found to be linked to the factors for these 
two characters. Later Punnett discovered linkage in a second group of 
factors consisting of those for green axils, cretin flower shape, and sterile 
anthers. Gregory has described a group of five factors in the Chinese 
primrose, those for red stigma, red flower color, long style, dark stems, 
and light corolla tube. In garden peas, Vilmorin and Bateson have both 
reported linkage between the factors for round as opposed to wrinkled 
seed and tendrilled as opposed to non-tendrilled or ‘‘acacia”’ leaves, and 
Hoshino has suggested coupling between red flower color and a factor 
for late flowering. Very recently O. E. White has investigated or com- 
piled the data on thirty-five factor differences in Pisum and has presented 
data for four linked groups of characters. In the garden snapdragon, 
Baur found linkage between the factors for red flower color and for the 
“picturatum” type of color pattern in the flower and also clear cut 
evidence of linkage between some other factors. Surface has shown that 
in oats the factors for pubescence on the back of the lower grain, pu- 
bescence on the back of the upper grain, and black grain color are closely 
linked. Enough cases have also been reported for other plants to demon- 
strate that linkage relations are of general occurrence in plants. 
In animals, Castle and Wright have suggested that linkage occurs in 
rats between the factors for red eye-color and pink eye-color. <A clear 
case also has been established by Tanaka in the silkworm moth in which 
a series of factors for larva pattern are linked to factors for yellow and 
white cocoon color. Besides these cases there are a large number of cases 
of sex-linked inheritance in many animal forms. These will be discussed 
in Chapter XI. 
It is clear from what we have stated above that aside from our 
knowledge of linkage in Drosophila, we have not progressed far in the 
investigation of linkage relations. Several factors have contributed to 
this condition., Most of the forms which have been used in genetic in- 
vestigations have a larger number of chromosomes than Drosophila, a 
fact which considerably complicates such investigations. Most genetic 
data have been obtained from experiments which involve but few fac- 
Digitized by Microsoft® 
