GENETIC STUDIES OF BABBITS AND RATS. 27 



steel-gray offspring in various matings, and this shows that they did 

 inherit darkened extension, E', from their mother. Their sire, as 

 we have stated, did not transmit E', but was of the formula Ee. 

 The steel-gray Flemish buck 2, used in crosses with both Polish and 

 Himalayan does, was also heterozygous for dark extension, his 

 formula being E'E. By Himalayan does (EE) he had 8 steel-gray 

 and 9 gray offspring (see table 33, VI). By Polish does (EE or Ee) 

 he had 10 steel-gray and 6 gray young (table 33, V). Unlike 9 7, 

 he had no black young. He was homozygous for the gray or agouti 

 factor (A) whereas 9 7 was probably heterozygous for this factor. 

 Hence the two black young of 9 7, her formula being E'EAa, but 

 that of cf 2 being E'EAA. 



These results confirm the conclusions of Punnett and show that — 



(1) Any individual which is homozygous for darkened extension is 

 devoid of agouti ticking, whether or not the agouti factor is present. 

 Examples are found in black Flemish and black Siberian rabbits. 

 (2) Any individual which is heterozygous for dark extension (E'E or 

 E'e) will ordinarily be steel gray in color ("agouti-black," Punnett) 

 if the agouti factor is present, either heterozygous or homozygous, but 

 such individuals are often black (showing no trace of agouti ticking) 

 in their first coat, although they develop the ticking in later pelages. 



However, some individuals of formula E'EAa may show little or 

 no ticking in their adult pelages, as, for example, the Fi 9 2474 and 

 her brother c?2473, which produced steel-gray young as well as 

 black ones when mated with each other. If the allelomorph of E 

 present in a heterozygous individual is e rather than E, the steel-gray 

 is usually (perhaps always) clearly visible in the adult coat. Punnett 

 speaks of individuals of this sort as invariably "black," but his 

 classifications were apparently based exclusively on the juvenile coat. 



In the F2 generation of the Flemish-PoUsh cross, but not of the 

 Flemish-Himalayan cross, appeared dilute pigmented individuals 

 having dark extension. These I have called "steel-blue " and "blue." 

 They correspond with the classes steel gray and black of the intense- 

 pigmented series. From the fact that no dilutes appeared in Fj 

 from the Flemish-Himalayan cross, it is clear that dilution was 

 introduced in the Polish race, but not in either of the other races. 

 Similar reasoning shows that the color yellow (e) also was not present 

 in either the Flemish or the Himalayan individuals employed 

 in the crosses, but only in the Polish individuals; for yellow F2 

 individuals appear in the Polish-Flemish cross, but not in the 

 Himalayan-Flenaish cross. Finally, angora coat appears in Fj of 

 the Polish-Flemish cross, but not in either of the other crosses. 

 Even in the Polish-Flemish cross angora appears only in the F2 of a 

 single mating, that between Flemish 9 7 and Polish cfS. But <f3 

 was employed also in the Himalayan-Polish crosses, yet no angora 



