Wright: Color Inheritance in Mammals 



427 



rodents than to recessive yellows such 

 as found in guinea-pigs or rabbits. 



A very different kind of dilution of 

 coat and eye color in rats was discovered 

 by Whiting.' In his red-eyed dilute 

 rats, the yellow of the agouti band is 

 reduced entirely to white, while the 

 black is relatively little affected. He 

 pointed out its very similar character 

 to the red-eyed dilution of guinea-pigs 

 and tests, which he made, soon showed 

 that it is also genetically similar in 

 being an allelomorph of albinism. As 

 in guinea-pigs, he found it to be im- 

 perfectly dominant over albinism, but 

 recessive to the intense color of wild 

 grays. It is, of course, to be placed in 

 class lb. 



THE HOODED PATTERN 



Probably the most interesting experi- 

 ments with rats are those dealing with 

 the hooded pattern. This pattern is 

 much less apt to be asymmetrical than 

 the piebalds in most other mammals. 

 Its variations can be arranged prac- 

 tically in a single linear series. White 

 first appears in small patches on the 

 feet and belly, creeps up the sides until 

 the typical hooded pattern is reached — ■ 

 a black head and narrow mid-dorsal 

 stripe, then obHterates the latter and 

 gradually reduces the black on the 

 head. Doncaster recognized four kinds 

 of rats with regard to pattern — self, 

 Irish a, Irish b, and hooded. The 

 Irish rats are nearly self with white on 

 the belly. He found that hooding was 

 a clear-cut recessive to the others but 

 that the Irish rats were of two sorts 

 genetically. Some of them (class a) 

 bred essentially hke self rats, never 

 producing pi ebalds . In these the ventral 

 white appeared to be a fluctuating 

 variation independent of the hooding. 

 Other Irish rats (class b), usually with 

 more ventral white than class a, pro- 

 duced 50% hooded rats in crosses with 

 the latter and were evidently heterozy- 

 gotes between self or Irish a and hood- 



ing. Castle^ obtained similar results 

 and carried on a very extensive experi- 

 ment on the effects of selection on the 

 hooding pattern, the results of which 

 have called forth a great deal of dis- 

 cussion. By long continued selection 

 the means of a plus and minus series 

 have been carried far beyond the 

 extremes of variation in the original 

 stock, approaching the condition of 

 Irish rats on the one hand, and black- 

 eyed white on the other. In the course 

 of the experiment a big advance ap- 

 peared suddenly in two rats of the plus 

 series, and at once behaved as a unit 

 Mendelian dominant to the stock from 

 which it rose. Rats from this source 

 were called the mutant stock. Other- 

 wise progress was very slow and uniform. 

 The results obviously have a most 

 important bearing on the efficacy of 

 selection in evolution, or in live-stock 

 breeding operations. Most discussion, 

 however, has centered on the inter- 

 pretation of the means by which selec- 

 tion has worked. There are really two 

 problems here. It has been widely 

 held that new variations are so rare 

 that they can virtually be neglected in 

 interpreting such an experiment.^ On 

 this view, selection has worked mereh" 

 by sorting out favorable combinations 

 of factors already present in the original 

 stock. Professor Castle, on the other 

 hand, has held that the results could 

 only be explained satisfactorily on the 

 view that minute variations occur 

 frequently if not almost invariably in 

 the production of germ cells. His idea 

 of the powers of selection is thus in 

 essential agreement with Darwin's. A 

 second problem is on the nature of the 

 genetic basis of the different grades 

 of the hooded pattern. On any view, 

 a large part of this variation is not 

 genetic but merely developmental. This 

 is shown by the low correlations between 

 parent and offspring. The weighted 

 average is +.23 in plus series and -J-.18 

 in minus series for sixteen and seventeen 



7 Whiting, P. W. 1916. Set. iV. 5., 43: 781. 



s Castle, W. E. 1912. Amer. Breed. Mag., 3: 270-282. 1915. Amer. Nat., 49: 37, 49: 

 713-726. 1916. Loc. cit. 1917. Aw.er. Nat., 51: 102-114. Castle, W. E., and J. C. Phillips. 

 1914. Pub. Cam. Inst. Wash. No. 195, 56 pp. 



3 See for example, Muller, H. J. 1914. Amer. Nat., 48:567. MacDowell, E. C, 1916. 

 Amer. Nat., 50:719-742. 



