15-i PROCEEDINGS OF THE AMERICAN ACADEMY. 



VII. COXCLUSIONS. 



From the experiments here recounted, the following conclusions as to 

 the heredity of color characters in mice are drawn : 



(1) Albinism. Complete albinism is a character recessive toward 

 either partial or complete pigmentation. Heterozygotes, from a cross of 

 pigmented with albino individuals, produce gametes having the albino 

 character and those having the pigment-forming character in approxi- 

 mately equal numbers. The chance combinations of gametes from two 

 such heterozygotes give rise, in the long run, to pure dominants, hetero- 

 zygotes, and recessives, in the proportions, 1 : 2 : 1. The pure dominants 

 and the pure recessives breed true inter se to the pigment-forming and the 

 albino characters respectively. A heterozygote bred to an individual of 

 either pure class produces, as an average result, equal numbers of hetero- 

 zygotes and pure animals. 



Partial albinism is"a condition in which the pigment is reduced around 

 definite body centres, so that unpigmented areas occur between the pig- 

 ment patches or at their borders. Ten such centres may be distinguished 

 in the mouse^ arranged symmetrically five on either side of the median 

 plane. These centres represent a cheek patch, a neck patch, a slioulder 

 patch, a side, and a rump patch. Various degrees in the reduction of 

 the pigment patches occur, or the patch may be eliminated altogether. 

 Partial albino, or pied individuals, breed true to that condition, but par- 

 tial albinism behaves toward complete albinism as a recessive character, 

 although the heterozygote may sometimes show an intermediate con- 

 dition. Moreover it is possible for albino mice to transmit either the 

 character total pigmentation or partial pigmentation, or an albino may 

 be heterozygous with respect to these two characters so that, when bred 

 to a pied mouse, equal numbers of pied and of self-colored offspring 

 result. 



When pure self-colored mice or rats are bred to pied animals having 

 albinism recessive, generation F^ consists of two sorts of pigmented 

 individuals both of which are heterozygous, one with respect to the 

 characters total and partial pigmentation, the other with respect to total 

 pigmentation and albinism. A random mating of the Fi individuals 

 results in the production of pigmented and albino offspring in the ratio, 

 15:1. A like result is obtained if the parental generation consists of a 

 pure pied animal and a totally pigmented one having albinism recessive. 

 Generation Fo, obtained by interbreeding the offspring of such parents, 

 consists theoretically of six classes of individuals, viz. : D, DR, DDR, 



