August 21, 1908] 



SCIENCE 



251 



by Bateson") that in the pigmentation of 

 guinea-pigs, three diiierent kinds of pigment 

 are produced, viz., black, brown and yellow. 

 In wild guinea-pigs (Cavidae) these pigments 

 are so placed on the individual hair as to give 

 it a banded appearance, and the banding is 

 inherited as a factor independent of the colors 

 present. In tame guinea-pigs this pattern 

 factor may be wanting, together with one or 

 more of the fundamental color factors, and 

 this loss of color factors gives rise to a long 

 series of color variations. But any mating 

 which will bring together in one individual 

 all of the four color factors will result in a 

 return (reversion) to the coat condition of the 

 wild Cavidxe. 



In the issue of Science for August 30, 

 1907, I showed that, if the hypothesis of an 

 independent color pattern (barring of the 

 hair) is correct, it should be possible to pro- 

 duce a color variety of guinea-pig at that 

 time unknown, one similar to the cinnamon- 

 agouti variety of mice. In confirmation of 

 the hypothesis, I may now say that this variety 

 has recently been produced, and in the follow- 

 ing way: Agouti-colored individuals were 

 crossed with chocolates. The young were all 

 agouti-colored. But when mated with each 

 ■other these agouti young produced offspring 

 of four sorts, agouti, black, cinnamon-agouti 

 and chocolate. The cinnamon agoutis are a 

 sharply defined and unmistakable new variety, 

 differing from the wild (agouti) type in the 

 total absence of black pigment from the eye, 

 the skin of the extremities, and from the hair. 

 The black young obtained from this cross, in 

 generation F„, were an unpredicted result 

 ' which serves further to confirm the hypothesis 

 of independent factors. 



Let us now apply the hypothesis to the 

 facts observed. The original agouti parents 

 hy hypothesis carry the four factors: (1) 

 black, (3) brown and (3) yellow pigments, 

 and (4) the barring pattern (agouti), and 

 have completely the wild type of pigmenta- 

 tion. The chocolates, however, have hair en- 

 tirely devoid of black pigment and unbarred. 

 They lack, therefore, the factors black and 



""Proc. Zool. Soc. Land., 1903. 



agouti (barring). 



In crosses of chocolate with agouti indi- 

 viduals, agoutis only are obtained, as already 

 stated, the presence of the factors black and 

 agouti dominating their absence. Using sym- 

 bols, B for black, Br for brown, Y for yellow 

 and A for agouti, the parental contributions 

 in this cross are: by the agouti parent, 

 BBrYA; by the chocolate parent, BrY. 

 The young, therefore, are heterozygotes of the 

 formula, B BrY A- BrY. Such individuals 

 should, in accordance with Mendelian prin- 

 ciples, produce ripe germ cells of four sorts: 

 viz., (1) BBrYA, (2) BBrY, (3) BrY A 

 and (4) Br Y. These four sorts should, on 

 the theory of probabilities, be equally numer- 

 ous. Each sort, if united with a germ cell 

 having the same constitution as itself, should 

 produce a different color variety, these four 

 varieties being, respectively, (1) agouti, (2) 

 black, (3) cinnamon-agouti and (4) chocolate. 



The result should be visibly the same if a 

 gamete united with one of another sort con- 

 taining fewer factors than itself, but none of 

 them different from its own factors. Thus 

 the first sort of gamete should produce an 

 agouti individual if united to either of the 

 other three sorts. Allowing for such unions 

 in their chance frequencies, we should expect 

 the second generation offspring to consist of 

 four visibly different sorts of individuals, on 

 the average, in the following proportions; 

 agouti, 9 ; black, 3 ; cinnamon-agouti, 3 ; choco- 

 late, 1. Up to the present time there have 

 been obtained, of agouti, 8; black, 4; cinna- 

 mon-agouti, 2; chocolate, 2. This is a per- 

 fectly normal Mendelian result, both qualita- 

 tive and quantitative, and confirms in the 

 most complete manner the hypothesis of an 

 independent pattern factor. For, can a more 

 severe test of the hypothesis be conceived than 

 that by its application one should produce a 

 wholly unknown variety? 



A moment's consideration of this case shows 

 what a really great advance in the theory and 

 practise of breeding has been obtained through 

 the discovery of Mendel's law. Wkat a puzzle 

 this case would have presented to the biologist 

 ten years ago ! Agouti crossed with chocolate 



