440 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
all F, males are barred, but all F: females are black. In F, barred birds 
and black birds occur in both sexes. These curious facts, which have 
been repeatedly verified, suggest the occurrence of a vehicle of inherit- 
ance which is duplex in males but simplex in females. What this is we 
do not know. No chromosome has been found which has a distribu- 
tion of this sort in fowls, but it is possible that some chromosome 
component, or other cell constituent has such a distribution and may 
be the actual vehicle of inheritance in such cases. The most important 
character economically, which appears to be affected by some sex- 
linked factor in poultry, is fecundity. Pearl has shown that when 
reciprocal crosses are made between Cornish Indian Games, a poor 
breed for winter egg production, and barred Plymouth Rocks, a fairly 
Ww M B 
Gq 7 rp) 
Ww & 
” 
a 
Sea ED 
| 6 
d 
Fic. 93.—B and C illustrate Morgan’s idea of the linear arrangement of the 
genes in the chromosomes. A and D show how the composition of the chromo- 
somes is supposed to change as the result of the crossover. On the right, a pair 
of chromosomes, a, before; 6, during; and c, after a double crossover. (After 
Morgan.) 
good breed for winter egg production, the F, females in each case 
resemble the father’s race more strongly than the mother’s race as 
regards egg production. Pearl did not maintain, however, nor do his 
experiments suggest, that the inheritance of fecundity depends exclu- 
sively upon a sex-linked factor. Goodale, however, has not been 
able to confirm Pearl’s observations, in the case of Rhode Island Red 
fowls. He finds no evidence of superior influence of the sire in the 
transmission of racial fecundity. 
