394 THE BIOLOGY OF BIRDS 



these " blue " Andalusians pair together they yield in the 

 next (F2) generation, 25 per cent, blacks, DD, 50 per cent. 

 " blues," D(R), and 25 per cent, whites, RR, with splashes 

 of grey. It had long been known that " blue "Andalusians 

 did not breed true, that they would not yield more than 

 six " blues " to the dozen. But the reason for this was 

 not clear until Mendelian inheritance was re-discovered 

 about 1900. The " blue " Andalusians do not represent 

 a true-breeding race, they correspond to D(R)'s. To get 

 twelve blue Andalusians to the dozen it is only necessary 

 to cross D and R, the black and the white. 



It should be noted, perhaps, that the matter is rather 

 more complicated than we have indicated, for besides the 

 blue-splashed Andalusians there are blue self-coloured 

 Andalusians (W. A. Lippincott, 19 18). The condition in 

 the blues is due to the combined action of two factors, one 

 securing black pigment for every feather in the body, and the 

 other acting on the distribution of the black pigment in such 

 a way that a characteristic blue-grey appearance results. 



The third fundamental idea in MendeHsm is that of the 

 segregation of factors in the germ-cells. When a fowl with 

 a dominant character is crossed with a fowl with a recessive 

 character, there must be in the fertilised egg-cell the factors 

 for both the dominant and the recessive characters. Let us 

 call these factors d and r. Only one of them, in typical 

 cases, becomes expressed in the body of the offspring. 



But the fertiUsed egg-cell gives rise to a lineage of germ- 

 cells which should by hypothesis all show the factors d and r. 

 What Mendel suggested, however, was that in the history of 

 these germ-cells there is a segregation into two sets, half of 

 them with the factor d and half of them with the factor r. 

 In other words, each germ-cell is " pure " with respect to 

 the factor of any particular unit-character. To put it 

 concretely, if there are 100 ova produced, 50 will carry the 

 d factor and 50 the r ; and the same for the sperms. If 

 the subsequent fertilisations are fortuitous, the Mendelian 

 ratio 1:2:1 must result in the offspring. 



Some Illustrations. — (i) Professor Punnett (191 9) gives 



