IN RATS AND GUINEA-PIGS. 7 



MENDELIAN INHERITANCE OF COAT-COLORS AND COAT-PATTERNS 



IN RATS. 



While pigment character and color-pattern are both inherited in Mendelian 

 fashion, the two are entirely independent of each other. They are separate 

 and uncorrelated unit characters. Accordingly we find that each type of 

 pigmented coat-pattern, namely, self, Irish, and hooded, may occur either 

 with gray or with black pigmentation, the frequencies with which they occur 

 in the respective combinations being governed by the laws of chance and of 

 Mendelian dominance. This will appear in the detailed discussion of the 

 experiments. 



Not only may each coat-pattern occur either in a gray or in a black 

 pigmented individual, but it may occur also in an unpigmented animal. Para- 

 doxical as this statement may seem, it is capable of abundant proof. The 

 coat-pattern of course is not visible in an unpigmented (albino) rat, but its 

 presence there as a potentiality can be demonstrated as certainly as the 

 occurrence of a recessive character in a heterozygous dominant individual. 

 Nothing but the presence of pigment is necessary to make the color-pattern 

 manifest. This can be supplied by a mating with a pigmented animal. 



Specific pigment potentialities (gray, black, or both) are likewise present 

 in albinos. Consequently we must recognize that albinos transmit inactive 

 both pigments and color-patterns; these, however, are unseen and can not 

 be made visible until some lacking substance borne by all pigmented indi- 

 viduals is supplied. Characters transmitted in this inactive state have been 

 termed by one of us (Castle, : 05) latent, and that terminology will be followed 

 in the present paper. 



ALBINISM RECESSIVE IN RELATION TO ALL TYPES OF PIGMENTATION. 



That total albinism behaves as a recessive Mendelian character has been 

 recognized independently by a number of investigators, among the earliest 

 being Correns (: 01) and Cuenot (: 02). The fact has been abundantly veri- 

 fied in the case of mice (see Castle and Allen, 103), rats (Crampe, '77-84; 

 Bateson, 103; Doncaster, :o6), rabbits (Woods, 103; Hurst :o5; and Castle 

 105), and guinea-pigs (Castle, 105). The experiments described in this 

 paper corroborate those of Crampe and of Doncaster with reference to rats. 

 The proportions of albinos and of pigmented individuals in mixed litters 

 are close to the Mendelian expectations, indicating neither selective union 

 of gametes nor lessened fertility of certain sorts of unions. Pigmented rats, 

 in which albinism was recessive, when mated inter se, have produced 129 

 albinos to 384 pigmented young, the numbers expected being 128 and 385, 

 respectively. Albinos mated with pigmented individuals, in which albinism 



