REVERSION IN GUINEA-PIGS AND ITS EXPLANATION. 



In 1905 I showed that when black-coated guinea-pigs of pure race are 

 mated with red-coated ones only black-coated young are ordinarily produced, 

 and that if such young are in turn mated with reds, both black young and red 

 ones are obtained. In other words, black is a mendelian dominant to red. 

 The fact was, however, noted that occasionally the cross of black with red 

 causes reversion to the agouti or wild type. But this may be regarded as a 

 modified condition of black, since the hairs of the agouti animal contain black 

 pigment, but disposed in a definite pattern with red, the entire hair being black 

 except a terminal or subterminal band of red (or yellow). 



An examination of the tables of matings published at that time shows that 

 all the agouti animals so produced were the progeny of a single red animal, 

 6^2054. This animal produced black young as well as agouti ones in crosses 

 with black, so it was not clear to what the reversion was due. By a study of 

 the progeny of this animal the matter was later cleared up. The black young 

 were found never to produce agouti young in crosses with any red animals 

 unless such reds were descended from 6^2054. The agoutis, however, produced 

 a mixture of agoutis, blacks, and reds, when mated with ordinary red animals. 



These and other corroborating experiments, reported briefly in 1907, 

 showed that the agouti reversion in crosses of black with red is due to a factor 

 transmitted by the red parent, never by the black one. For, as I then showed : 

 (1) a red animal which produces the reversion to agouti in crosses with one 

 black animal will produce it in crosses with any black animal ; but (2) no black 

 animal will produce agouti young unless crossed with a red animal which also 

 produces agouti young in crosses with other black animals. 



For simplicity, the something possessed by red animals which induces 

 reversion, I have called the agouti or A factor. It is invisible in the red animal, 

 since the hairs of such animals are red throughout their length. Its only dis- 

 coverable function is to exclude black from the terminal portion of the hair, 

 and this function plainly can not be exercised unless black pigment is present. 

 Now this agouti factor is transmitted like any other simple independent men- 

 delian factor. Some reds are homozygous in A and so transmit it in all their 

 gametes. A very fine female of this sort, kindly loaned me in the summer of 

 1906 by Mr. B. B. Horton, was mated with two different black males (cr4456 

 and cf 9538), by each of which she bore five young, all agouti-marked. These 

 same males when mated to other red females produced only black young. 



Most of the agouti-producing reds which I have had, including the original 

 cf 2054 (received in June, 1903) and some of his descendants, have been hetero- 

 zygous in the agouti factor, so that when mated with black animals they pro- 

 duced in approximately equal numbers agouti 3'oung and black ones. All 



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