To6 



IRISH GARDENING. 



must carry a black halt' and a white half, one 

 from each parent, thus : — 



O 

 But breed now with the half-and-half animals, 

 and what must happen ? It is a question of 

 chances. Both parents carrying- mixed deter- 

 minants, the black half of one may meet 

 with either the white half or the black half of 

 the other ; so also may the white half, and 

 then, g-iven a sufficient number of mating-s, 

 there are bound to be one pure white pro- 

 g-eny and one pure black one to every two 

 mixed ones. The following- diairram will show — 



This 

 Gives 



O 



X* 



o 



o 





o 

 o 



That is to sa}-, the blue fowl are bound to 

 leave young- in the proportion of one pure black 

 to one pure white to two hybrid blues ; and 

 the black ones will breed black when bred with 

 black, the white will breed white when bred 

 with white, and the hybrids will always breed 

 the three colours in the proportion i : 1:2. 



If a hybrid is bred with a pure one the young- 

 will be one half pure, one half hybrids, thus : — 



This S>2g Gives • S • S 



But this only explains how hybrids breed back 

 ag-ain, not how new varieties arise. Other 

 experiments do this, however. Determinants 

 do not always work as simply as they do in the 

 case of the black and the white fowl. For 

 instance, when black and red cattle are crossed, 

 the young- are not half way between in colour, 

 but are all black. The black colour is dominant 

 to the red, and g-ets its own way. When these 

 hybrids are crossed together ag-ain their young- 

 do not come out in the proper ratio - one red, 

 one black and two something- else — but in the 

 ratio one red to three black, the black colour 

 still dominating the red. But there is this 

 about them that two of the three blacks are 

 really something- else They are animals carry- 

 ing- both determinants, like the blue fowl above, 

 but with the black determinant dominating- 

 the red to apparent extermination. Yet the 

 red colour is not really exting-uished, it is merely 

 subdued,* for when the black-red hybrids are 

 bred tog"ether one red appears in every four. 



Let us put it g-raphically, using- letters instead 

 of circles, with capitals for the dominant colour 

 and small letters for the subdued. 



Thus "'^=='B 



Give 



B B B r 



B, r, r, r 



Mendel used Ihe word "recessive." 



That is : one pure black, one pure red, and 

 two that appear black, but are really black- 

 reds in which black is dominant to red. These 

 black-reds continue to reproduce in this way 

 for ever. 



But there is still more than this. Plants and 

 animals vary in more than colour, and in many 

 thing-s at the same time. Let us take examples 

 from animals because I am most familiar with 

 them. Let us suppose the black and red 

 animals above have another character in which 

 they differ. Let them differ in size — one breed 

 beingf stout and the other tall, stoutness 

 being dominant over tallness. Then in the 

 same way the hybrid generation should give 

 one stout, one tall, and two stout-tails that 

 appear to be stout, thus : — 



S S S t 



S, _ t, t, t 



Now, what is going to happen when we breed, 

 say, a tall black animal with a red stout one : 

 tallness and stoutness being dominant ? Taking- 

 colour alone, when we breed with the hybrids 

 we shall have, as we have seen above, one 

 black, one red, and two apparent black ones. 

 Add on the size variation to these three different 

 kinds and we get each of them reproducing 

 their own colour, but with the size varying 

 in the proportions of one stout, one tall, and 

 two apparent stouts. Thus: — 



The Blacks — 



The Black-reds 

 (eight instead 

 of four) — 



The Reds- 



Let us arrange them symmetrically and we 

 shall see the result more clearly : — 

 BB BB Br Br 



SS St SS St 



Those underlined are pure in respect to both 

 colour and size, the others are all hybrids in 



