December 18, 1914] 



SCIENCE 



905 



p. 60). The chief point of interest in this 

 investigation was the fact that the simultane- 

 ous presence in the zygote of R, the factor 

 for rose comb, and P, the factor for pea comb 

 produce an entirely new character, namely, the 

 walnut comb. Two walnut-combed birds pro- 

 duced by a cross of pea comb X rose comb 

 gave, when crossed together, an F„ progeny 

 consisting of walnut, rose, pea and single 

 comb, in a ratio of 9, 3, 3, 1. 



A similar result would be obtained if the 

 parents used in the original cross were walnut 

 comb of the formula REPP and single comb 

 rrpp. 



In this last case if we focus our attention on 

 the walnut comb we should see that it recurred 

 in approximately 9 out of 16 of the P, progeny. 



A character dependent solely upon one 

 mendelizing factor is present in three fourths 

 of the F, progeny. The ratio of those lacking 

 it to those having it being as 1:3. When, 

 however, two factors are needed for the mani- 

 festation of a character, as in the case of the 

 walnut comb, the character is lacking in a far 

 greater number of P„, namely, in 7 out of 16. 

 The ratio of those lacking the character in 

 question to those having it becomes 1 : 1.3 in- 

 stead 1:3, as in the case involving only one 

 factor. 



If three factors are necessary for the mani- 

 festation of a given character, the F„ ratio 

 shows a still greater proportionate increase of 

 animals lacking the character. If the simul- 

 taneous presence of factors A, B and is 

 necessary for the manifestation of a given 

 character, the number showing the character 

 in F„ may be calculated as follows : F„ will be 

 made up of 27ABO, 9ABc, 9AbC, 9aBC, 3abC, 

 3aBc, 3 Abe, labc. Only the 27 ABC animals 

 will show the character question, and the ratio 

 of those lacking the character to those having 

 it will be as 1.3 : 1. 



An actual cross of this sort is the follow- 

 ing: a wild black agouti mouse having the 

 factors B for black, A for agouti and D for 

 intensity was crossed with a dilute brown 

 mouse having the factors b for brown, a for 

 non-agouti and dil for dilution. 



Fi animals were all Aa Bb Ddil, all of them 

 having the character in question, namely, in- 

 tense hlach agouti pigmentation. 



When these Fj animals are crossed together 

 they should give a ratio of 27 intense black 

 and 140 other colors, while the expected num- 

 bers obtained were 107 intense black agouti 

 and 140 other colors, while the expected num- 

 bers are 105.3 intense black agouti and 141.7 

 other colors, respectively. 



Another cross with mice recorded by Phillips 

 and the writer^ (1913) will serve to illustrate 

 the case of four factors. Here the ratio ex- 

 pected is one animal having the character in 

 question, to 2.16 lacking it. 



From Table I.^ it will be seen that there are 

 in F„ 436 animals possessing the character 

 in question (intense black agouti) to 744 lack- 

 ing it, the expected numbers being 373 to 807. 



As the number of factors increases, the ratio 

 of animals which do not show the character to 

 those that do increases rapidly. 



With 10 factors it becomes 16.7 : 1, with 15 

 factors, 73.8 : 1, and with 20 factors 314.3 : 1. 



It will be convenient to present this in tabu- 

 lar form as follows : 



The general principle involved is that, with 

 the addition of each factor involved, the num- 

 ber of F„ animals possessing the character in 

 question is multiplied by three, while the total 

 number of F^ zygotes is multiplied by four. 

 It will be seen, therefore that the difference 

 between the number of animals with the char-. 



2 Little, C. C. (1913) and Phillips, J. C, "A 

 Cross Involving Four Pairs of Mendelizing Char- 

 acters in Mice, ' ' Am. Nat., Vol. 47, pp. 760-762. 



3 Loc. cit., p. 761. 



