1921.] 



Eesearch in Animal Breeding. 



255 



(3) The approximation of the Fl birds to the weight of 

 the larger parent. 



(4) The great variation in weight shown by the F2 generation. 



(5) The production in F2 of birds larger than the Hamburgh, 

 and of others smaller than the Sebright. 



For if the Hamburgh were AABBCCtJd, and the Sebright 

 aahhccDD, the Fl birds must all be AaBhCcDd. They will be 

 uniform, and at the same time, since they contain a dose each 

 of 4 factors, they will not on our hypothesis be much lighter than 

 birds w^hich, like the Hamburgh, contain a double dose of 3 

 factors. When, however, such birds are bred together they 

 should give an F2 generation showing great variation, for such 

 Fl birds should produce germ cells of 16 different kinds with 

 respect to the four size factors involved, viz. : — 



From the meeting of two such series of germ cells it is clear 

 that all sorts of sizes will result; but the reader who wishes to 

 follow out these possibihties in detail must be referred to the 

 original paper.* It should, however, be noticed that such a com- 

 bination as AABBCCDD will occur, in which a bird is pure for 

 all 4 factors. Birds of this combination, as well as others, such 

 as AABBCCDd or AABBCcDD, should be heavier than the Ham- 

 burgh. Again, we may have the combination aahhccdd in wte-h 

 none of the 4 factors are found. Such birds must be smaller 

 than the Sebright. 



The theory is in accordance with the series of facts to be ex- 

 plained that was set out on pp. 253 and 255. It can, how^ever, be 

 subjected to further test. The very large F2 birds, and the very 

 small ones should, on the theory, breed true to size. Lack of 

 opportunity prevented the testing of the biggest ones, but a pair 

 of the smallest F2 birds (show^n on Plate 1, Fig. 2 and 3) w^as 

 mated together, and found to breed true to the unusually small 

 size. Lastly, among the birds of intermediate size there should 

 be some which are pure for 2 factors, e.g., AABBccdd , w^hich 

 should breed true to a size intermediate between that of the 

 Hamburgh and the Sebright. Recent tests have revealed the 

 existence of such birds. 



This series of experiments suggests that even so complicated 



m 



** "On Inheritance of Weight in Poiiltrv," hy R, C. Punnett and 

 P. G. Bailey : Journal of Genetics, Vol. IV, IDU.^ 



ABCD 

 ABCd 



ABcD 

 ABcd 



AhCD 

 AhCd 

 AhcD 

 Ahcd 



aBCD 

 aBCd 

 aBcD 

 aBcd 



ahCD 

 ahCd 

 ahcD 



a bed 



