358 Doubleness in Stocks 



which are present in all the strains, there appears to be a second pair 

 of factors (X'Y'), the presence of which also renders the zygote single. 

 One member of this second pair appears to occur in some but not all 

 of the pure single strains, the other in some but not all of the double- 

 throwing strains. 



(9) The effect of the coupling mentioned under (7) is that when 

 only the pair of factors common to all the strains is present {XY), the 

 mixed F^ families from a cross between the non-double-throwing 

 pure-bred and the eversporting single contain a proportion of about 

 3 s. : 1 d. 



(10) When a similar cross is made between forms which contain in 

 addition one member of the second pair of factors mentioned under (8), 

 the distribution of the members of the second pair being complementary, 

 some of the mixed F^ families again show a proportion of about 3 s. : 1 d., 

 but in others the proportion of singles is considerably higher. 



(11) White plastids result from the presence of a factor {W), 

 cream plastids from absence of the same factor {w). 



(12) Pure white or cream races are homozygous in W and w 

 respectively, but the sulphur-white race is heterozygous in regard to 

 this factor which is present in some only of the ovules and absent 

 altogether from the pollen ; moreover in this latter race W appears to 

 be coupled with one of the factors required for singleness. W^e may 

 represent the gametic series in this sulphur-white race thus: 



Ovules Pollen 



n — 1 XYW all xyw 



1 ZyF 



I xYw 

 n — 1 xyw 



(13) The distribution of the allelomorphs W and w among the 

 gametes of F-^, where the parents are of unlike plastid colour, appears 

 to depend upon the conditions under which the plastid colour factor is 

 introduced into the cross, ie., whether by the male or the female germ, 

 and whether in combination with singleness or doubleness. 



(a) When the union is between two eversporting forms, and when 

 W is introduced on the female side in combination with XY {XYW 

 ovules) and w on the male side with xy {xyw pollen) as in the cross 

 d-white $ X f^cream ^, all the F^ pollen appears to carry creamness 

 {w) as well as doubleness {xy) like the pollen of the ^ parent : and 



