IO4 Experimental Zoology 



keeps on growing. At three months it is 6 to 9 cm. long. If 

 shedding now takes place the animal never acquires longer hair; 

 but animals that reach the age of four months without shedding 

 their longest hairs may ultimately acquire hair 14 to 16 cm. long. 



It has been stated that long hair is recessive to short hair. 

 When two long-haired individuals are mated, having hair of 

 different lengths, the offspring have hair like that of the shorter- 

 haired individual. In the next generation both long- and 

 short-haired individuals occur, but the number of long-coated 

 individuals exceeds the Mendelian expectation. 



A pure albino, long-haired, and rough-coated individual, was 

 crossed with nine different pure, pigmented, short-haired, and 

 smooth females. Twenty-nine young were produced, all pig- 

 mented, short-haired, and rough-coated. Thus while two of 

 the dominant characters come from the smooth-haired father, 

 the third comes from the rough mother the rough coat dis- 

 sociating from the long hair to influence the arrangement of the 

 short hair of the progeny. 



Castle and Forbes have recently extended Castle's earlier 

 experiments with short- and long-haired guinea pigs. When 

 crossed the short hair dominates, but the hair of the hybrid is 

 nearer the upper limit of variation of the pure, short-haired type, 

 showing perhaps the influence of the other character. When 

 the hybrids were inbred not only the two grandparental types 

 reappeared, but a new intermediate type was present. This 

 type contained individuals of all intermediate grades between 

 the long- and short-haired types. It contained, first, individuals 

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

 but much more slowly than does the long hair, and shows a ten- 

 dency to break off at lengths much less than that of the long hair. 

 The long hairs were less numerous, as if some of the hairs only 

 were continuous in growth, while the other hairs ceased to grow 

 after a time. The alternative characters behaved somewhat 

 like a mosaic. In other individuals the hair ceased to grow, 

 i.e. was determinate in growth, but not until it had reached a 

 length of 60 to 80 mm. These cases seemed to be a blend, but no 



