392 MENDEL'S LAW AND THE HEREDITY OF ALBINISM. 



all alike, and raises the question whether all are equally pure as regards the pigment- 

 forming character. 



Construed in the strictest sense, the doctrine of gametic purity is untenable. 

 We cannot accept that interpretation of it which requires that the gametes formed 

 by an individual be the precise equivalents, in all particulars, of the respective gametes 

 which united to form that individual. Mendel himself would not have assented to 

 such an interpretation, for in the latter part of his original paper ('66) he clearly states 

 the important principle that a composite character may undergo resolution into its 

 elements in consequence of crossing. This allows a part of a complex character to 

 pass into one gamete, while the remaining parts pass into another; in other words, 

 it makes possible the formation of mosaic gametes, into whose composition the domi- 

 nant and recessive characters may both enter in varying degree. All gametes which 

 contain any portion of a dominant or of a recessive character associated with its oppo- 

 site, are in reality mosaic; yet if a gamete essentially recessive contains only traces 

 of the dominant character, it may be convenient to recognize this fact in its desig- 

 nation, which we do by calling it an impure recessive. 



In guinea-pigs the impurity of recessives tainted with the dominant character 

 is commonly visible. Ordinary white guinea-pigs with pink eyes, though they in- 

 variably produce albinos when bred inter se, have a greater or less amount of sooty 

 black pigment in the skin and hair of their ears, nose, and feet, showing the presence 

 of a trace of the dominant character. Rarely is it possible to obtain an animal free 

 from this visible taint, and even when obtained, we are informed by breeders, such 

 individuals are likely to produce offspring with a certain amount of pigment on their 

 ears or feet. 



The so-called Himalayan rabbit is another illustration of a mosaic with a pre- 

 dominantly recessive (albino) character, in which the dominant (pigment-forming) 

 character is localized precisely as in the impure albino guinea-pig, namely, at the 

 extremities. Himalayan rabbits have brownish-black noses, ears, feet, and tails, 

 being elsewhere snowy white and having pink eyes. They breed true inter se, yet, 

 according to Darwin ('76, p. 114), may occasionally produce a silver-gray animal, 

 in which the pigment is not restricted to the extremities. This condition must result 

 from liberation of the dominant character from the strict localization which it has in 

 ordinary individuals and which it must have also in the gametes that produce them. 



In mice, on the other hand, impurity in recessive individuals is not visible, though 

 doubtless sometimes present. So long as the breeder wishes only to obtain white 

 mice, it makes no difference what the ancestry of his breeding animals is. All albinos 

 alike will produce only white offspring when bred to albinos. But if the breeder 



