THE CAUSE OF ALBINISM AND DOMINANT 



WHITENESS. 



Bv H. ONSLOW 



For some time biologists have been aware that 

 white animals may be divided into two distinct 

 classes : albinos, or recessive whites, and dominant 

 whites. These two types are visibly, and were 

 hitherto chemically indistinguishable, being capable 

 of differentiation only by means of the offspring 

 which the}' produce. The recessive whites, or 

 common albinos, are widely distributed among 

 domesticated animals (e.g., albino mice, rabbits, 

 guinea- pigs, and so on) : they have pure white fur 

 and pink eyes, which appear totally devoid of 

 pigment, but which, on microscopical examination, 

 are found to contain minute pigment granules. 



Let us suppose an albino rabbit is mated to a 

 self-black rabbit ; all their offspring will be the 

 same colour as the coloured parent — that is to say, 

 they will all be black. If these black offspring of 

 the first, or F lf generation are mated together, they 

 will have black offspring and white offspring in 

 the ratio of three black to one white, in accordance 

 with the simple Mendelian law. The white offspring 

 will, of course, be pure recessives, like their albino 

 grandparent, and will produce nothing but albinos 

 when mated with each other, whereas two thirds of 

 the black offspring will be heterozygous for albinism 

 like their parents, and the other third pure black like 

 their self-black grandparent. 



The second type of white animal — dominant 

 white — is superficially indistinguishable from an 

 albino, except that the eyes are usually more 

 heavily pigmented, and occasionally there are 

 flecks of pigment upon the body. Both types of 

 white have a correlated piebald form, in which the 

 white portions of the pattern are respectively 

 recessive and dominant to self-colour. Now, if 

 a dominant white rabbit is crossed with a self- 

 black, all the offspring will be white, instead of 

 black as in the case of the albino ; and, if the 

 dominant white is only heterozygous for white- 

 ness, half the young will be heterozygous dominant 

 whites like one parent, and the other half will be 

 self-blacks like the other parent. It will thus be 

 seen that albinos are recessive to colour, and 

 dominant whites dominant to colour, as the name 

 signifies. 



The problem, therefore, was to discover what 

 physical difference underlay this variation in 

 genetic behaviour. 



It has been shown that dark animal pigments, 



* During the process of extraction the organic peroxide component of the peroxidase was usually, but not invariably, 

 destroyed in these experiments. It can, however, be easily replaced by a very small quantity of hydrogen peroxide. 



f For this purpose so-called " English " rabbits were used. They are a black-eyed white rabbit with a few coloured 

 spots on the back and flanks. 



I This reaction was not as strong as the previous ones, owing to the fact that the ferment fluid was diluted by as 

 much as an equal volume of the recessive white extract. 



or melanins, are in many cases the result of the 

 interaction of an oxydase and a chromogen. The 

 oxydase is generally a tyrosinase, and the chromogen 

 either tyrosine or a closely allied substance. By 

 the union of an organic peroxide and a peroxidase 

 an extremely unstable third substance is formed, 

 which on decomposition gives its oxygen to the 

 colourless chromogen, and thereby converts it into 

 a melanin. 



It was found possible to obtain a colourless fluid, 

 rich in tyrosinase,* by making an extract from the 

 skins of very young black rabbits. By adding to this 

 extract a few drops of a saturated aqueous solution 

 of tyrosine as a substitute for the natural chromogen 

 (which appears to be destroyed, together with the 

 organic peroxide, during the process of extraction), 

 and incubating at blood-temperature for twelve 

 hours, the mixture becomes oxidised. A deep 

 black ring forms at the surface of the fluid, where 

 it is in contact with the atmosphere, indicating 

 the formation of melanin. Now, if a fluid is 

 extracted in an exactly similar manner from reces- 

 sive and from dominant white rabbits, f and if 

 it is incubated with tyrosine in the same way, 

 no darkening occurs. This shows that either the 

 tyrosinase is absent, or, if it is present, its action is 

 inhibited by some other substance. In order to 

 test this, varying quantities of the two white 

 extracts were incubated with an equal quantity 

 (two cubic centimetres) of an active tyrosinase 

 solution, tyrosine, and hydrogen peroxide. The 

 results were as follows : — 



+ + Indicates strong reaction. 

 + Indicates positive reaction. 

 — Indicates no change. 



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