92 PROcr>:EDiNGs of the American academy. 



This range of 4 individuals in the theoretical total is due to the fact tliat 

 in three, or possibly four, cases the litters were so small that they did 

 not show conclusively whether or not the parents were producing gam- 

 etes having the albino character. For it is obvious that where both 

 parents are pigmented, the expectation is but one albino in four young, 

 provided both parents have the albino character recessive, lience where 

 less than four young are obtained, and no albinos are among them, it can- 

 not be told, from this test alone, whether one or both of the parents pro- 

 duce the two sorts of gametes. For if one parent produces only gametes 

 having the pigment character, no albino young will result even though 

 the other pai'ent produces the two sorts of gametes (formula 5). The 

 three cases of this sort among von Guaita's crosses (Nos. 8, 21, 22) neces- 

 sitate making a possible allowance of from .5 to 1.5 individuals in the theo- 

 retical total. Another litter (No. 23) for which allowance may be made 

 is one the parents of which were a spotted and an albino mouse. Had 

 the former contained the albino character in a recessive condition, the 

 theoretical result would have been 50 per cent of pigmented and 50 per cent 

 of albino individuals. To be sure, all of the four young resulting from 

 the cross are pigmented, yet in the tables previously given one sees that 

 it is possible to obtain in a single litter, from a cross of this sort, four pig- 

 mented individuals with no albinos. Hence, if the greatest possible allow- 

 ance be made, the actual result is but 2 individuals short of the theoretical, 

 while the most favorable allowance would give an exact agreement. 



It may be well to explain in brief the methods by which the above 

 conclusions as to the expected projtortions of albinos were reached, and 

 also to analyze the results still further. Generation P (the original 

 parents crossed — Bateson) consisted entirely of crosses in which the 

 one parent (the spotted mouse) possessed only the dominant character 

 and the other (the albino mouse) only the recessive character. In all 

 cases, therefore, by formula (1), heterozygotes are produced, and no 

 albinos. The second generation (Fj) was obtained by interbreeding 

 these heterozygotes (Fj). Assuming that segregation of pigment and 

 albino characters takes place in accordance with Mendel's law, it is ex- 

 pected (by formula 2) that 25 per cent of the resulting offspring will be 

 albinos. The fours pairs of heterozygotes (Nos. 2-5) did actually produce 

 albinos except in one case (No. 4), where but four young were born. In 

 a total of 44 individuals of this generation there were actually 14 albinos, 

 an excess of 3 over the theoretical number. Generation F;, consists 

 of the young of five pairs selected from the preceding generation. The 

 first of these pairs was of two albinos, and only albino young resulted. 



