572 



THE AMERICAN NATURALIST [Vol. LVI 



Orloff male X Plymouth Rock female gives cocks closely re- 

 sembling the true Plymouth Rock, that is, crossbarred with slow 

 feathering development. All the hens, however, are black (since 

 in the Plymouth Rock there is also a gene for melanism, * ' tif a, ' ' 

 which is not sex-linked), and they develop feathers quickly. 

 ^: asuke atuge atrage atife X ?: suke tuge trage tifa 



F, J* : suke tuge trage tifa $ : asuke atuge atrage tifa 

 In F2 the coupling between suke, tuge and trage becomes 

 broken, and dilferent new combinations are to be observed in 

 rather large numbers. More often the forms asuke-tuge-trage 

 are obtained, colored like Plymouth Rock, but with quick de- 

 velopment of feathering (among these there are also cocks), and 

 conversely suke-atuge-atrage, with slow feathering, but black 

 (when tifa is present). In one case a suke-tuge-atrage chick 

 appeared, with slow feathering and silvery, but not crossbarred. 



In the light of the Morgan theory these facts can be explained 

 by regarding the genes suke, tuge and trage as being in a sex 

 chromosome which cannot give crossing-over in the heterozygous 

 sex (female). But when the same chromosome is transmitted 

 to the Fi male, it undergoes crossing-over with its partner, which 

 occurs most often in the space between suke on the one side and 

 tuge-trage on the other. Crossing-over between suke-tuge on 

 the one side and trage on the other occurs less often, wherefore 

 the arrangement of the genes in the F^ may be represented as 

 follows : 



suke tuge trage 



However, the counts of chicks which have so far been obtained 

 in F2 are not yet large enough to ascertain definitely the order 

 of the genes, and therefore still less the exact distances. 



[Crossing-over between ''suke" (barring) and "tuge" (sil- 

 very) has also been announced by Goodale (1917) and by Hal- 

 dane (1021), in tlie papers listed below, whieli wore not available 

 to the above author. 



A. S. Serebrovsky 



TE OF Experimental Biology, 

 Moscow, August 21, 1922 



Goo.lnle, 11. 



Haldano, J. 

 p. 663, 



Note of Transmitter. 11. J. Muller.] 



