REYERSION IN GUINEA-PIGS AND ITS EXPLANATION^ 



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

 mated with red-coated ones only black-coated young are ordinarily produc 

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

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

 The fact was, however, noted that occasimi^Uy_the_crQ§g„M.ilafik_wi;^ 

 causes reversion to the agouti or wild typeT But this may be regarded a 

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

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

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



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

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

 cf 2054. This animal produced black young as well as agouti ones in cros 

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

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

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

 unless such reds were descended from cf 2054. The agoutis, however, produ( 

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



These and other corroborating experiments, reported briefiy in 19 

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

 transmitted by thered parent, never by the fetact one. For, as I then show 

 (XT' a'red ahTmal which produces the reversion' to agouti in crosses with i 

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

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

 produces agouti young in crosses with other black animals. 



For simplicity, the something possessed by red animals which indu 

 reversion, I have called the a^utijorjjactor. It is invisible in the red anin 

 since the hairs of such animals are.xed throughpuj: their length. Its only c 

 ^verSBTe function is to exclude black from the^ terminal portion of thejij 

 and this function plainly caSTnoTTSe" exercised unless black pigment is prese 

 Now this agouti factor is transmitted like any other simple independent m 

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

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

 1908 by Mr. B. B. Horton, was mated with two different black males (c?'4' 

 and 0^9538), by each of which she bore five young, all agouti-marked. Th 

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



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

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

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

 duced in approximately equal numbers agouti young and black ones. 



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