274 Shull. 



The F; females benig- heterozygous for the broad leaf will produce 

 two kinds of eggs, XBF and XhF, and the Fi males will produce two 

 kinds of sperms, XBF and Xbf, thus permitting four combinations when 

 crosses are made among the Fi plants. The F2 zygotes produced from 

 these combinations will be as follows: 



XBF. XBF =: homozygous broad-leafed females, 

 XhF . XBF = heterozygous broad-leafed females, 

 XBF. Xbf = heterozygous broad-leafed males, 

 XbF . Xbf = homozygous narrow-leafed males. 



These hypotheses regarding the genetic constitutions of the Fo 

 plants, formed a working basis for genetic experiments involving these 

 plants. The following points are to be emphasized: namely, (a) that 

 although there were only three phenotypes present in the Fa, there 

 were four genotypes; (b) that half of the F2 females must be heterozygous 

 and half homozygous with respect to the gene B; and (c) that all the 

 broad-leafed males must be heterozygous for the gene B. To test these 

 inferences a genetic analysis was made of all the broad-leafed plants 

 of my F2 generation. For the sake of clarity the results of these analyses 

 will be presented in four sections, based on the nature of the germ- 

 cells which were brought together in each particular mating. The 

 series of crosses which form the basis of these four sections were as 

 follows: (1) Every female was fertilized by one of its narrow-leafed sibs. 

 As the females were genotypically of two kinds the results fall into 

 two groups, (a) and (b), according as the females were heterozj'gous or 

 homozygous for the factor B; (2) All the broad-leafed males were crossed 

 with one or moi'e of their female sibs, and as some of the females 

 chosen for the crosses were homozygous, others heterozygous, this series 

 of crosses also falls into two groups, (c) and (d). A remarkable and 

 wholly unexpected situation in regard to the sex-ratios will be noticed 

 in these tables. They present an independent pi'oblem wliich must not 

 be permitted to confuse the otherwise perfectly consistent results of all 

 these matings. 



(a) Crosses between heterozi/cjous females and their narrow-leafed sibs. 



On the supposition that the heterozygous females of the F» have 

 the formula XBF. XhF, and the narrow-leafed males XhF. Xbf, there 

 should be again two kinds of eggs, XBF and XbF and two kinds of 

 sperms, XbF and Xbf, giving the four following combinations in equal 

 numbers: 



