56 LINKAGE 
the mother. The explanation of this result is obvious; 
for the son gets his single X chromosome from his 
mother, and should therefore have the characters 
that go with this chromosome. His Y chromosome, 
derived from the father, does not influence the result 
at all. The daughters, however, get one X chromo- 
some from the mother (yellow white) and the other 
from the father (gray red). The factors for gray 
and red dominating give gray red daughters. 
The composition of these F,; females can be tested 
by breeding to the double recessive male (yellow 
white) since this does not carry any dominant factors 
which will obscure what factors are received by the 
F, females from their mothers. But the F, males 
are themselves yellow white, so that the F, females 
may be mated to their brothers. In fact, the out- 
come is the same, whether a yellow white male from 
stock or a yellow white F, brother is bred to the F, 
female. The F, offspring of such crosses give the 
following classes and ratios: 
Non-crossovers Crossovers 
Yellow white Gray red Yellow red Gray white 
49.5 per cent. 49.5 per cent. 0.5 per cent. 0.5 per cent. 
99 per cent. 1 per cent. 
This F, result reveals the kinds of eggs produced by 
the F, female (since a double recessive father was 
used). Crossing over takes place between yellow 
and white in only 1 per cent. of cases. 
There is no way of testing linkage in the F,; male, 
which is like a homozygous individual so far as the re- 
