DISCOVERY 



201 



Offspring. 



Fowls, black X splashed 

 white (ist generation) 



Fowls, black x splashed 

 white (2nd generation) 



Black 



White 



Fi Andalusian 



Factors. 

 BB 



Reproductive Cell 

 All B 



B + 6 



bb 



B6 



Fi Andalusian B6 



B6 



BB 



B6 

 B6 



b + b 



Appearance. 



All Andalusian 

 ( = Fi) 



Black (25%) 1 



Andalusian 



(50%) 

 White (25%) 



(=F2). 



proportions in which the different sorts occur. In 

 subsequent generations the F2 whites will breed true 

 like the original pure white strain, the blacks breed 

 true like the original black, and the Andalusians will 

 give the same results as their Fi Andalusian parents. 



Another character in which breeds of fowls may 

 difier is the shape and size of the comb. If two 

 breeds, one with the ordinary high comb (so-called 

 single comb), the other with a rose comb (a low comb 

 with httle warts on it), are crossed, we shall get results 

 precisely similar to those found in the Andalusian 

 fowl. The second, or F2, generation will consist of 

 25 per cent, of birds which will breed true to single- 

 comb shape, 25 per cent, which will breed true to 

 rose-comb shape, and 50 per cent, which will contain 

 one factor for each of the two comb-shapes, and there- 

 fore will not breed true. 



The question immediately arises, What will happen 

 if a white bird with a single comb is crossed with a 

 black bird with a rose comb ? Will the white colour 

 and single-comb shape stick together, or shall we be 

 able to separate and recombine them ? The answer 

 given by experiment is that recombination is possible. 

 In the F2 generation, we shall get two new pure- 

 breeding types — white birds with rose combs, and 

 black birds with single combs — as well as t}'pes like 

 the original parents, and types that do not breed true. 



Similar results have been obtained by the hundreds 

 of thousands of crosses which have been made during 

 the last twenty years with Mendel's principle in mind, 

 and we can assert, wdthout any fear of contradiction, 



that for the majority of characters which go to make 

 up any species or race of animal or plant, and to mark 

 it off from other species, the fundamental laws laid 

 down by Mendel hold good. 



To be brief, we may say, first, that all the characters 

 of a species are controlled in their development by 

 definite units or factors, which can be handed on 

 from parent to offspring. Secondly, that all such 

 factors are present in pairs, one being derived from 

 the father, the other from the mother. Thirdly, that 

 when the reproductive cells are fonned the two 

 members of a pair separate from each other, so that 

 each sperm or each ovum must contain one member 

 of a pair, but cannot contain both ; and, finally, that 

 different unit-factors are inherited independently of 

 each other, so that, by making the right series of 

 crosses, we can build up new races or varieties of 

 animals by combining factors for characters of different 

 existing breeds. All kinds of characters have been 

 shown to be inherited by means of unit-factors — 

 colour, shape, size, fertility, vigour, resistance to 

 disease, abnormahty, length of life. The external 

 environment may often modify the way in which a 

 character develops ; but the basis which can be modi- 

 fied at aU, and the limits of the extent to which it can 

 be modified, are provided by the factor and its influence 

 during development. 



Through Mendel, the idea of the unit-factor was 

 introduced into the study of inheritance. By the 

 labour of innumerable workers, some of the best-known 

 of whom are Bateson and Punnett in England, Baur 



I Fowls, black and rose 

 I combx splashed white 

 I and single comb (ist 



generation) 

 I Ditto (and generation) 



B.R + b.r - 



Andalusian, rose 

 ( = Fi). 



Thus, besides combinations which will not 

 breed true, we shall get BR + BR = pure for 

 black and rose, like one grandparent ; b .r -\- b .r 

 = pure for white and single, like the other 

 grandparent ; and also B . r -1- B . »- = pure for 

 black and single ; and 6 . R -t- 6 . R = pure for 

 white and rose — i.e. two new pure-breeding 

 strains. 



' These fowls appear with a rose comb, ^ rose is what is caUed dominant to single ; i.e. rose appears even when only one R is 

 present. Rr birds, therefore, appear rose-combed, but do not breed true for comb-shape. 



