410 CARNEGIE INSTITUTION OF 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. 3-13.) 



During the past year further progress has been made in attacks on 

 some of the central problems of genetics, viz: (1) What characteristics 

 of mammals are inherited? (2) Do these conform with Mendel's 

 law; if not, with what law? (3) Are Mendelian characters quantita- 

 tively variable; if so, is their variation controllable through selection or 

 otherwise? 



On the material side we have made good progress in the experimental 

 breeding of guinea-pigs, rats, and rabbits, large numbers of which have 

 been reared from pedigreed stock. In this work and in the study of the 

 records obtained, I have had valuable assistance from Messrs. Sewall 

 Wright and H. D. Fish. One of the important results of the year's 

 work is the demonstration that multiple allelomorphs are of quite 

 general occurrence among manmials. By this it is meant that a unit- 

 character may assume several different alternative forms. For exam- 

 ple, in guinea-pigs normal pigmentation and albinism are Mendelian 

 alternatives (allelomorphs) . This has long been known, but what has 

 not been known is the fact that two intermediate stages of pigmentation 

 are allelomorphs of these and of each other. These new conditions 

 are dilute pigmentation and a still lower grade of pigmentation in which 

 the fur contains only black or brown pigment, without yellow. The 

 four conditions form a graded series of variations in one and the same 

 Mendehan unit-character, in the production of which only a single 

 genetic factor is involved, so far as we are able to discover. It follows 

 that Mendelian factors are subject to quantitative variation, if our 

 interpretation is correct. That the case is not an isolated one is shown 

 by the occurrence of similar variations in other factors, both in guinea- 

 pigs and in other mammals. Thus the agouti factor is subject to a 

 series of allelomorphic variations in mice and rabbits as well as in 

 guinea-pigs; and the extension factor and white-spotting factor also 

 manifest multiple allelomorphic conditions. It would seem that this line 

 of evidence must render untenable the presence-and-absence hypothesis 

 in accordance with which Mendehan characters were supposed to be 

 either present or absent, but not to vary otherwise. 



Through a study of crosses between Cavia cutleri from Peru and 

 domesticated guinea-pigs, further hght has been obtained on the vexed 

 question of size inheritance in mammals. Size inheritance has been 

 correctly described as "blending," but a Mendehan interpretation 

 suggests that this may be due to multiple independent factors, and 

 invokes as evidence the greater variability of the F2 generation as com- 

 pared with the Fi generation. Material of sufficient genetic constancy 



