Skitember 25, 1903.] 



SCIENCE. 



39y 



bearing: magenta red and white flowers 

 respectively as well as other plants bear- 

 ing lavender flowers. The numerical pro- 

 portions observed in two successive seasons 

 are shown in Table I. The observed num- 

 bers, it will be seen, are quite close to the 

 theoretical 1:2:1. 



In cases wherein the hybrid is indis- 

 tinguishable from one of the parent forms, 

 t. e., in cases of complete dominance of 



TABLE II. 



Heredity of Cotyledon-color among Crossbred 



Peas. 



Offspring. 



Gen. III. Gen. IV. 



Y(G)- 



f2Y(G). 

 ilY 



G I ...G 



IG I ...G 



j,2Y(G); 



t-lY [ ...Y 



Y ! ...Y 



one parental character, only two cate- 

 gories of ofl:"spring will be recognizable 

 and these will be numerically as 3 : 1. But 

 further breeding will allow the separation 

 of the larger group into two subordinate 

 classes— first, individuals bearing only the 

 dominant character; secondly, hybrids; 

 that is, into groups A and A(B), which 

 will be numerically as 1:2. 



Observed results are in this case also 

 very close to theory. Jlendel, by cross- 

 ing yellow with green peas, obtained, as 

 we have seen, only yellow (hybrid) seed. 

 Plants raised from this seed bore in the 

 same pods both yellow seed and green seed 

 in the ratio 3:1. (See Table II.) Under 

 self-fertilization, the green seed produced 

 in later generations green seed only. It 

 bore only the recessive character. Of the 

 yellow seeds, one in three produced only 

 yellow ofi'spring, i. e., contained only the 

 dominant character; but two out of three 

 proved to be hybrid, producing both green 

 and yellow seed, as did the hybrids of the 



preceding generation. These are precisely 

 the theoretical proportions, A-|--2A(B) 

 + B. 



In the case of mice, it has been shown 

 independently by Cuenot ( :02) and by 

 the writer's pupil, Mr. (J. IM. Allen, that 

 the second hybrid generation, obtained by 

 crossing gray with white mice, consists of 

 gray mice and white mice approximatel.y 

 in the ratio 3:1. (See Table III.) The 

 white are pure reeessives, producing only 

 white offspring, when bred inter se. Wliat 

 portion of the grays are pure dominants 

 has not yet been determined with precision, 

 but we may confidently expect that it will 

 prove to be not far from 1 in 3. 



TABLE III. 



Heredity of Cdat-coi.or among Cross-bred Mice 



obtained by mating white mu'e (\v) 



WITH Gray Mice (G). 



A further test of the correctness of 

 STendel's hypothesis of the purity of the 

 germ-cells and of their production in equal 

 numbers, is afforded by back-crossing of 

 a hybrid with one of the parental forms. 

 For example, take a case of simple domi- 

 nance, as of cotyledon-color in peas or 

 coat-color in mice. We have here char- 

 acters D (dominant) and R (recessive). 

 The first generation hybrids will all be 

 D(R). Any one of them back-crossed 

 with the recessive parent will produce fifty 

 per cent, of pure recessive offspring and 

 fifty per cent, of hybrids. 



For the hybrid produces gcmi-cells D -|- R 

 The recessive parent produces germ- 

 cells R-fR 



The possible combinations are.... 2D(R) -|- 2R 



