THE BOOK OF POULTRY. 



are the impure roses in F, each individual must 

 be mated to a single-comb bird : the pure rose 

 bird will give a progeny all rose-comb. The 

 reason is obvious when the pure dominant is 

 mated to the recessive, the resulting progeny all 

 resemble the dominant, in this case all are 

 rose-combed, thus : — 



the impure mated to the recessive will give hah'' 

 rose and half single, thus : — 



From this we gather how to test for the purity 

 of any one character in a bird. Mate the doubt- 

 ful bird to a bird known to be pure recessive in 

 that character, the progeny will decide the ques- 

 tion, all being pure if both are pure, some 

 recessives appearing if the bird under analysis is 

 impure. 



The two cases (pea and rose-comb) investi- 

 gated are examples of nearly perfect dominance 

 where the F, generation resembles the parent 

 which supplies the dominant factor. Domin- 

 ance has been stated to be an essential part 

 of Mendelism. This is an error, for the 

 dominance is often imperfect, but this imperfec- 

 tion does not interfere with the application of 

 Mendelism. The blue colour of the Andalusian 

 fowl is an example of imperfect dominance. 

 The blue colour is due to the mixture of black 

 and white (excellent blue plumage can be pro- 

 duced by putting white chickens in a brooder 

 with a smoking lamp, but they are not " stay- 

 blues"). When blues are bred with blues, the 

 progeny consists of black, blues, and splashed 

 white in Mendelian proportions : i black ; 2 

 blues ; i splashed white. On further breeding, 

 blacks mated to blacks give only blacks, whites 

 mated to whites give only whites, but blacks 

 mated to whites give blues. The blue is a 

 hybrid-character formed by the union of black 

 and white, and in Mendelian terms the blue bird 

 is said to be heterozygous. 



Blue X Blue Black X Spla>hed White 



In the above case the hybrid-character is 

 easily distinguishable from either the dominant 



or recessive, whilst in the two preceding cases it 

 could only be distinguished by the test of 

 breeding. 



What is meant by this term ? Dominance is 



the name given to an observed fact that one 



character, owing to the " presence " of 



Dominance some factor, appears in the individual 



to the exclusion of another character 



(recessive) which is unable to appear. 



The numerical proportions given throughout 

 this chapter hold good only when large num- 

 bers are bred. With small numbers 

 Numbers chances are against their being 



found to be absolutely true, but 

 when sufficient numbers are taken the actual 

 results appro.ximate closely to the theoretical. 

 So far we have only considered one allelomor- 

 phic pair at a time, and we have found the 

 proportion to be 3D : iR. If two allelomorphic 

 pairs are considered, the result is more com- 

 plicated and the possible combinations are 

 considerably extended, as each pair is trans- 

 mitted independently of the other. The fol- 

 lowing experiment will illustrate what is meant. 

 A Campine cock was crossed with a Silkie hen 

 and from the many allelomorphic pairs in the 

 birds we will select for this illustration comb 

 and feathers, though any two could have been 

 selected without prejudice to the argument. 

 The Campine has single comb and feathers, 

 the Silkie a rose-comb and silk. The F, birds 

 were all rose-comb and feathered, single comb 

 and silk being recessive. Let R = rose-comb, 

 s=single, F= feathers and y=silk. Then the 

 Campine cockerel will be Fs and the Silkie hen 

 will be Ry : the zygotic combinations of Fj 

 birds will be RsFy. Taken independently, any 

 allelomorphic pair gives rise to four possible 

 zygotic combinations in F^. In F., the four 

 zygotic combinations of the first allelomorphic 

 pair, Rs, will be RR, Rs, Rs, ss ; likewise those 

 of the second pair, Fy, will be FF, Fy, Fy, yy. 

 It is clear since the pairs of allelomorphs are 

 separately transmitted that the first of the four 

 zygotic combinations of the first allelomorphic 

 pair RR can appear with every one of the four 

 zygotic combinations of tlie second pair, FF, Fy, 

 Fy, yy and will give rise to four combinations 

 RRFF, RRFy, RRFy, RRyy. Similarly the 

 remaining three combinations of the first pair 

 (Rs, Rs, and ss) can also each combine with 

 each of the combinations of the second pair. 

 There are thus sixteen possible combinations : — 



(RR -f- Rs + Rs -I- ss) (FF -I- Fy -)- Fy -h yy) 



These results are made evident by such 

 a diagram as the following, in which the 

 four zygotic combinations of the first pair are 



