94 PROCEEDINGS OF THE AMERICAN ACADEMY. 



meiited, hence in order that the albino class shall be represented in their 

 progeny it is necessary that both mice of each of the three pairs shall 

 be heterozygotes, having the albino character recessive. This condition 

 evidently obtained in case of pair 1, for out of a total of 32 young, 10 

 were albinos, where but 8 were expected. Neither of the two other pairs 

 produced albinos, but their totals are too small to show conclusively the 

 nature of the parents, the one pair having but two, the other three young. 

 Pair 2 of this generation consisted of two albinos, and as exj)ected, all 

 their young were of that class. Pair 3 consisted of a spotted and an 

 albino mouse. The former evidently had the albino character recessive, 

 for the result of the cross with the albino mate was to produce 23 pig- 

 mented and 20 albino mice. This result is less than two individuals short 

 of the expected equality of the albino and the pigmented classes. 



Finally, for generation F,; there are three litters, one of which was 

 destroyed before its character could be determined. Of the remaining 

 two litters, one consists of 3 albinos from albino parents, the expected 

 result. The other pair is of a spotted and an albino mouse. In this 

 case no albinos are expected provided the spotted parent is pure as 

 regards the albino character. As a matter of fact, none of the four 

 young obtained was an albino. Nevertheless it is within the limits of 

 possibility, as the writer's experiments have shown, to obtain no albinos 

 in a litter of four, even when 50 per cent of the young are expected to 

 be of this sort. Hence it is necessary to make allowance for the possi- 

 bility that the spotted parent may have been a heterozygote containing 

 the albino character recessive. 



The crosses made by von Guaita, as far as they throw light on the 

 heredity of complete albinism, may be summed up as follows: (1) In all 

 cases wherein pure pigmented (spotted) animals were mated with albinos 

 the pigmented character of the former proved dominant over total albin- 

 ism. (2) Unions of two albinos resulted in the production of albino 

 offspring only. (3) In cases where both parents were proved to be 

 heterozygous with respect to the two characters of coat color, 43 albinos 

 to 117 pigmented mice were produced, the albinos being thus 3 individ- 

 uals, or less than 2 per cent, in excess of the theoretical number. If the 

 three indeterminable cases be added, the number of albinos is but one 

 individual less than that expected. (4) In cases where it is expected 

 that the two classes shall be as 1:1, this ratio is very closely approxi- 

 mated, the totals being 24 pigmented : 20 albino mice. It is clear, then, 

 that von Guaita's mice behaved according to the Mendel ian expectation, 

 and as far as known to the writer, his experiments are the earliest pub- 



