322 



BIOLOGY. 



BIOLOGY. 



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

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

 reports see Year Books Nos. 3-20.) 



During the past year further progress has been made in the study of the 

 linkage relations of genes in mice, rats, and rabbits. No new linkages have 

 been discovered, but more precise values have been found for those already 

 reported and the relative frequency of crossing-over in the two sexes has been 

 more fully investigated. 



In mice the linkage relations of seven genes have been investigated, two of 

 which are linked, being located, as we suppose, in the same chromosome. 

 The seven genes are (1) agouti, (2) albinism, (3) pink-eye, (4) chocolate, (5) 

 dilution, (6) piebald, and (7) black-eyed-white. The two genes which are 

 linked are albinism and pink-eye and the cross-over percentage between them 

 is for male parents 13.7 ±0.4, and for female parents 16 ±0.6. The numbers 

 of young on which these averages are based are males 7,007, females 3,345, 

 numbers large enough to make the probable errors very small. Accordingly, 

 it may be regarded as certain that in mice crossing-over occurs a little more 

 freely in oogenesis than in spermatogenesis. The same' thing is found to be 

 true in rats. 



In the case of rats (Mus norvegicus), the linkage relations of five genes have 

 been studied, viz, (1) agouti, (2) albinism (and its allelomorphs), (3) pink- 

 eyed yellow, (4) red-eyed yellow, and (5) hooded pattern. Of these five 

 genes, three are found in one linkage system, presumably lying in one and the 

 same chromosome. They are albinism and the two kinds of yellow. Albinism 

 is very closely linked with red-eyed yellow, but neither of them is linked any- 

 thing like as closely with pink-eyed yellow. In all cases the cross-over per- 

 centage is higher for female parents (i. e., in oogenesis) than for male parents 

 (spermatogenesis), as the following figures show. 



The order of the three genes, on the linear arrangement theory, is evidently 

 (1) albinism, (2) red-eye, (3) pink-eye. The total "map-distance" would be 

 for females 21.9, for males 18.4, a complication not encountered in Drosophila, 

 where only female parents give cross-overs. 



In rabbits the linkage relations of six genes have been rather fully investi- 

 gated. The investigation of three others is in progress. The six genes on 

 whose linkage relations adequate information is in hand are (1) agouti, (2) 

 albinism (and its several allelomorphs), (3) dilution, (4) yellow, (5) English 

 spotting, and (6) angora (long) hair. Of these, the last two are linked with 

 each other, the cross-over percentage being 16.0 ±1.7 in the case of male 



