440 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



all Fi males are barred, but all Fi females are black. In F2 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 



cm 



M 



O 



ur 



B 



M 



a 



Fig. 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; h, during; and c, after a double crossover. {After 

 Morgan.) 



good breed for winter egg production, the Fi 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. 



