26 GENETIC STUDIES OF RABBITS AND RATS. 



It can therefore be stated with confidence that no linkage is found 

 between large or small size and the four coat-characters, albinism, 

 dilution, yellow, and angora. A reason for this has already been 

 suggested, that each of the coat-characters is determined by a single 

 recessive gene borne probably in a single chromosome, and in the 

 case of each of the four characters in a different chromosome, whereas 

 size depends on many genes borne probably in many different chro- 

 mosomes or in all the chromosomes. 



In table 33 is recorded the color of each rabbit studied, but 

 an examination of these records reveals nothing essentially new as 

 regards color inheritance. It does, however, serve to confirm the 

 discovery made by Punnett (1912) of a peculiar form of the extension 

 factor (E) which he observed in the offspring of a certain Himalayan 

 rabbit. To be sure, Punnett did not call it a peculiar form of the 

 extension factor, but rather a darkener (D) inseparably coupled with 

 the extension factor (E), so that the supposed couplet, darkened 

 extension (DE of Punnett), behaved as the allelomorph of ordinary 

 extension (E) and of yellow (e). A series of 3 allelomorphs was thus 

 established by Punnett's observations, and it would seem desirable 

 for simplicity so to designate them. I have elsewhere employed 

 the symbol £" for Punnett's darkened extension (DE). The order of 

 dominance of the 3 allelomorphs is E' (dark extension), E (ordinary 

 extension), e (restriction or yellow). 



Our observations on the Flemish crosses reveal the probable 

 source of Punnett's peculiar darkened extension found in his Himala- 

 yan doe 7. No Himalayans of pure race, that we have had, possessed 

 the darkened extension, but it is regularly present in Flemish Giant 

 rabbits of the varieties steel gray and black. Doubtless Punnett's 

 Himalayan doe 7 was derived from a Flemish cross made with the 

 idea of intensifying or darkening the pigmented markings of the 

 Himalayan (nose, ears, tail, feet). 



All our pure Flemish rabbits have possessed darkened extension. 

 The black doe, B, was apparently homozygous for this factor. She 

 transmitted it to two young, 1:^2473 and 9 2474, which she had by 

 the Polish male 3, whose formula was Ee. These two young are 

 recorded as black, but as full-grown adults it was noted that each of 

 them at times showed slight indications of ticking on the neck or 

 front legs, and among their offspring were typical steel grays (e. g., 

 9 3108, 9 3111, 0^3420), similar in appearance to 9 7 (plate 1, f). 

 That animal was heterozygous for dark extension, her formula 

 being E'E. Mated with the Polish buck 3, she had a litter of 9 

 young, of which 5 were gray, 2 steel-gray, and 2 black. The 5 gray 

 never produced any steel-grays when mated inter se, which shows 

 that they did not inherit darkened extension, but only ordinary 

 extension, E. But the steel-gray and the black young produced 



