Belling: Lethal Factors and Sterility 



163 



for example, was to be expected in the 

 following different proportions. 



7/2 : //// : /'2 



Fertile plants (?»-+w-) : ■imn : iin-+n') 



Semi-sterile plants. 2mn : 2(w-+«-) : 2mn 



With a gametic ration of 5:1, which 

 approximates the figures actually found, 

 the plants would be in the following 

 proportions (7). 



Gametic ratio, 

 5:1 or 1:5 



H2: nil -.112 



Fertile plants 



Semi-sterile plants 



26 : 20 : 26 

 10 :52 :10 



Dom.: Rec. 



1.8 :1 

 6.2 :1 



The fertile plants have a large excess 

 of homozygotes, and the semi- sterile 

 plants a corresponding excess of hetero- 

 zygotes. The results for coupling are 

 the same as those for repulsion. 



In the case of Oenothera lamarck- 

 iana, which Geerts (16) has proved to 

 be semi-sterile in pollen and embryo- 

 sacs, we may use the same hypothesis 

 as for the Stizolobium crosses; in which 

 case the fertile combinations, K2I2 and 

 k2Lo, perish as embryos, and only the 

 semi-sterile plants, KkLl, survive, the 

 proportions in the second generation 

 with regard to any factor B which is 

 linked with L being, for either coupling 

 or repulsion, 



Bi-.Bb-.hn 

 mn: (;n^+w^) '.mn. 



This gives a large excess of hetero- 

 zygotes. We get precisely the same 

 result as regards linkage if we take up 

 the alternative (and perhaps, more 

 probable) hypothesis of a pair of 

 allelomorphs, Aa, one of which is 

 lethal to the pollen grains which con- 

 tain it, and the other lethal to the 

 megaspores having it. In this case, 

 however, a separate factor pair, C\ and 

 Co, must be regarded as responsible for 

 the death of half the embryos. 



In the double-throwing stock (Mat- 

 thiola) which gives about equal num- 

 bers of singles and doubles, the facts 

 (40) may, I think, be met by the hy- 

 pothesis that this plant, in addition to 

 being heterozygous for a factor E 

 (double plants having ^2), differs from 

 the normal wild fomi in that the factor 



E has undergone a mutation to E\ E^ 

 being lethal to pollen grains. Then any 

 factor B (white-flower color as opposed 

 to cream) linked with £', will give, in 

 the progeny of the heterozygote E^eBh, 



Plants with B2 : Bb : hi 



Normal flowers mn : m'^-{-n^ : mn 



Neuter double flowers. . . w- : 2mn : m'' 



This gives (with close coupling) many 

 Bb and very few B2 and 60 plants among- 

 the singles, and mostly 62 plants with 

 iewBb among the doubles (40). 



Some extensive investigations of the 

 mortality of pollen grains in known and 

 suspected hybrids have been made 

 lately, especially by Jeffrey (21 and 22), 

 Dorsey (13), Standish (43), Hoar (19), 

 and Cole (10). Further work of a 

 quantitative nature on the amount 

 and inheritance of this mortality, and 

 on the state of the embryosacs of these 

 hybrids, is needed before a factorial 

 hypothesis can be applied. There are 

 several distinct causes for empty pol- 

 len grains (and for aborted embryosacs, 

 which, however, are not readily count- 

 ed). (1) There is a mortaUty due to 

 accidents of environment; in which 

 case the lethal effect is usually different 

 in different flowers on the same plant, 

 or in different plants of the same 

 homozygous line, or at different times 

 of the year. Cold, at a critical period 

 of pollen formation, in the spring or 

 fall, affects the pollen of some tropical 

 plants; as Stizolobitmi, or cotton (1). 

 This mortality is apparently not selec- 

 tive, and presumably does not affect 

 the ratios of zygotes. (2) There is a 

 partial mortality of pollen grains due 

 to zygotic factors, which factors I have 

 found cause the death of usually a 

 small fraction of the pollen grains in 

 certain fertile and semi-sterile lines 

 from vStizolobium crosses. This tend- 

 ency is inherited, but is apparently 

 random, not selective. (3) The whole 

 (or nearly the whole) of the pollen of 

 a plant may perish by the action of 

 zygotic factors, as in the sweet-pea with 

 empty anthers. In these cases the 

 abortion is not selective. (4) Lethal 

 factors, or combinations of factors, 

 acting on the haploid generation may 

 cause semi-sterility; that is, the death of 

 half the pollen grains and half the em- 



