MENDELISM 437 



3 Green round (squares 11, 12, and 15). 



5 Green wrinkled (square 16). 

 Or, briefly, 9 Y R, 3 Y W, 3 G R, i G W in every sixteen peas. 



Of the 9 Y R, only i (square i) is homozygous in both 

 respects, and should produce only Y Rs. Two Y Rs are homo- 

 zygous in colour only, but heterozygous in shape (viz. squares 



2 and 5); they should produce only yellow, but both " Rounds " 

 and " Wrinkleds." Tw^o Y Rs are homozygous in shape, but 

 heterozygous in colour (viz. squares 3 and 9) ; they should 

 produce only rounds, but both " Yellows " and " Greens." The 

 remaining 4 (squares 4, 7, 10, and 13) are heterozygous both in 

 colour and in shape, and should produce all 4 kinds — Y R, Y W, 

 G R, and G W. 



On the 3 Y W, i (square 6) is homozygous in both shape 

 and colour, whilst the remaining 2 (squares 8 and 14) are homo- 

 zygous in shape only. Similarly with the 3 G Rs, that in 

 square 1 1 is homozygous in both respects, the other 2 (squares 

 12 and 15) being homozygous in colour only. It wdll, of course, 

 be noted that the character in respect of which all the 3 YWs 

 and all the 3 G Rs are homozygous is a recessive one — namely, 

 wrinkledness in Y W and greenness in G R. 



There is only i G W zygote w^hich, since both its characters 

 are recessive, is therefore homozygous in both respects. 



A very close approximation to the result 9 Y R, 3 Y W, 



3 G R, I G W has been obtained in experiments by Mendel, 

 Bateson, Hurst, and myself. And the various types of Y R, 

 YW, and GR have also been recognised. 



Another point of view from which the proportion 9:3:3:1 

 may be regarded is from that of the zygotes solely. If we have 

 a generation which displays the two characters of one pair Y 

 and G in the proportion 3 Y to i G in every 4, and the two 

 characters of another pair R and W in the proportion 3 R to 

 I W in every 4, and if w^e further suppose that the tw^o pairs are 

 independent of one another, we should expect that in every 16 

 (= 4 X 4), 9 (= 3 X 3) w^ould be Y and R, 3 (= 3 x i) Y and W, 

 3 (= 3 X i) G and R, and i (= i x i) G and W. And we find 

 that this is so, w^hich shows that the tW'O pairs are independent 

 of one another. 



Having become familiar with the signification of the 9:3:3:1 

 proportion, let us proceed to the discussion of other results 

 which can only be interpreted in the light of it. We need not 



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