320 CARNEGIE INSTITUTION OP WASHINGTON. 



ZOOLOGY. 



Castle, W. E,, Harvard University, Cambridge, Massachusetts. Continua- 

 tion of experimental studies of heredity in small mammals. (For previous 

 reports see Year Books Nos. 3-16.) 



The past year has seen a large amount of experimental work accom- 

 plished with a minimum of assistance, but with no publications to 

 record. Withdrawal of students and former assistants to enter war 

 service has given more time for personal investigation, but has limited 

 that which could be given to preparation of material for publication. 

 The experimental work with guinea-pigs secured in Peru in 1911 has 

 been concluded. The single problem under investigation in mice has 

 made good progress, so that we are now able to state definitely and 

 quantitatively the previously demonstrated linkage between two 

 recessive characters in mice, albinism and pink-eye. The next step 

 is to look for other similar inter-relations among the genetic characters 

 of mice and thus lay a foundation for a knowledge of the genetic 

 architecture of the mammalian germ-cell. A parallel investigation 

 with rats is in an even more promising stage, for here three characters 

 in one linkage system have been demonstrated and the quantitative 

 linkage relations between two of them have been fully worked out, 

 and large stocks of rats have been bred which will soon give linkage 

 values with the third character, and will indicate whether the grouping 

 of genes is linear or not. A search for other linked genes in rats is 

 also being made. A large stock of rabbits is being bred for a fuller 

 investigation of size inheritance in mammals, a difficult problem 

 regarding which conflicting views are still held. Special precautions 

 are being taken to overcome the difficulties encountered in our pioneer 

 investigations in this field made some years ago and to clear up existing 

 uncertainties. The critical second-hybrid generation is now being 

 produced and will be of full adult size in about one year. A possi- 

 ble Hnkage relationship of color characters in rabbits is also under 

 investigation. 



The question of the stability of genetic factors and that of the 

 action of selection in relation to genetic change are finding satisfactory 

 solutions in current investigations. Several minor investigations with 

 rats and rabbits are concerned with these questions. 



A favorable opportunity for the study of inheritance in relation to 

 milking qualities of pure breeds of dairy cattle is being utilized. The 

 experiment involves a cross between registered Holstein and registered 

 Guernsey cattle. Full individual milk records have been secured for 

 the foundation stock used from both pure breeds and from the Fi 

 hybrids. The F2 generation will be milking next year. 



