HEREDITY OF HAIR-LENGTH IN GUINEA-PIGS. 



3. THE EFFECTS OF CROSS-BREEDING BETWEEN SHORT-HAIRED AND 

 LONG-HAIRED ANIMALS. 



(a) Mating BXD. 



When the short-haired animals were mated with animals of class D, 

 only young of the former sort were obtained, which result shows clearly 

 the dominant character of short hair. However, the maximal hair-length 

 of the cross-breds was usually 40 to 50 mm. (class B, Fig. 2) ; that is, it fell 

 within the tipper half of the range of variation of the pure short-haired 

 stock, though in no case, we think, did it transcend the range of variation 

 of the short-haired stock itself. We are warranted, then, in concluding 

 that the dominance in the offspring of the short-haired type of growth is 

 complete. Type B mated with type D gives only type B. 



(&) Mating B{D) XB(D). 



But when cross-breds between the types B and D were bred together, 

 not only did type D reappear after skipping a generation, but a new and 

 intermediate type was found also, which we may call type C. This is not 

 sharply separated from types B and D, but is made up of individuals scat- 

 tered all the way between those parental types. It contained, first, indi- 

 viduals whose hair grew continuously from the age of twenty days on, but 

 much more slowly than does the hair of class D, with a tendency to break 

 off at lengths much less than those attained in class D. The long hairs of 

 such animals were also frequently less numerous than in typical class D, 

 as if part of the hairs only were continuous in growth, while the others 

 were determinate. In these individuals we see the alternative characters 

 coexisting as in a mosaic. In other cases animals placed in class C appeared 

 to have only hair of determinate growth, but growth continued until a 

 length of 60 to 80 mm. had been attained before the hair narrowed to a base. 

 Such animals represent an intimate blend rather than a mosaic of the con- 

 trasted types B and D, yet with a closer approximation to type B; but no 

 sharp line could be drawn between the blends and the mosaics, as the two 

 graded into each other and into the two parental types. These intermediates 

 (class C) were about as numerous as the animals of type D, but only about 

 half as numerous as those of type B. Thus, in a particular experiment there 

 were produced twenty-nine B, twelve C and ten D individuals, together with 

 eleven of unknown character, because they died or were disposed of before 

 their hair was fully grown. The grandparents of this lot were the long- 

 haired male, 2002 (Castle, :o5, PI. i, Fig. i), and several different short- 

 haired females ; the parents were all of class B. The Mendelian expectation 



