UNIT-CHARACTERS (GENE MUTATIONS) 21 



different types and rates of pigment production in the hair 

 follicle, with an inhibitor for the black process when the 

 banded region is forming. Chocolate pigmentation which 

 takes the place of black in some fancy varieties may be 

 looked upon as a condition in which the black reaction has 

 been checked in an early phase. 



Albinism (c, mutant form of the color gene, C). 



The white mouse of Pontis is said to ruminate. — Aristotle, 300 B.C. 



Complete albinism (see Fig. 15) is a condition in which 

 pigmentation is entirely wanting in all parts of the body. Not 

 only are external organs devoid of pigment but even those 

 internal regions which normally develop pigment, such as 

 the eyeball and the outer surfaces of the brain and spinal 

 cord, are unpigmented. Animals bearing albinism have 

 white hair and pink skin. The eye color is usually either 

 pink or whitish according to whether or not the retinal blood 

 supply is visible through the iris. Histological examination 

 reveals the fact that in albinos pigment granules are present 

 which are normal in shape and distribution, but which are 

 leucotic. 



A common explanation (140) for the production of albi- 

 nism is the absence of the colorless tyrosinase, which ferment, 

 working upon the base or substrate, tyrosin, converts it into 

 the pigment melanin. Several chemical reactions are in- 

 volved in the process. Some maintain that it is the tyrosin 

 which is lacking in albinos. This assumption appears more 

 probable in view of the allelomorphic series of dilutions 

 produced by different forms of the albino gene. 



The gene or hereditary determiner for albinism in the 

 mouse is the lowest step in a series of alternative chemical 

 states possible for this particular gene. The other states of 

 this gene produce respectively normal pigmentation (C), 

 chinchilla (c ch ), and extreme (Himalayan) dilution (c 11 ). 

 A corresponding complete series of allelomorphs is found in 

 rabbits. Albinism in crosses (18, 69) behaves as a recessive 



