LETHAL FACTORS AND STERILITY 



Certain Factors Cause Death of Homozygous Embryos— Sublethal Factors or 

 Combinations Also Affect the Zygote — Action on Pollen Grains and Embryosacs 



John Belling, Washington, D. C. 



I. LETHAL FACTORS, OR COMBINATIONS 

 OF FACTORS, ACTING ON THE ZYGOTE 



A?\TOMINALLY recessive (actu- 

 ally intenncdiate) genetic factor, 

 causing the death of all embryos 

 homozygous for it, was demon- 

 strated by Baur, in Antirrhimun, in 

 1907 (2). On selfing, the heterozygote, 

 Aa, gave \A-^:2Aa:Oa2 {\ green: 2 

 gold-color). Similar factors have been 

 found in several other cases. In these 

 plants, the embryos burst the seed- 

 coats, and sometimes, as in maize, 

 have a sufficient food store to grow 

 leaves some inches long. But in Oeno- 

 thera lamarckiana, as Renner (37) has 

 shown, }/2 of the embryos die in the 

 seeds, without emerging. It is, I think, 

 likely that such a mortality in the seed 

 exists in other cases; probably in Vib- 

 morin's dwarf wheat (52) which was 

 constantly heterozygous, and gave ap- 

 proximately a ratio of OA'>'.2Aa:\a2 

 (two dwarf to one tall) when selfed; 

 and possibly also in many first-genera- 

 • tion species hybrids. 



In the heterozygote Aa, where a^ is 

 lethal, if any factor B is linked with A, 

 then a plant heterozygous for AB will 

 give progeny, as regards B, in the pro- 

 portion of {ni^-\-2mn)B2'. 2(m^-\-nin-\- 

 n^)Bh-\-{n'^-\-2nin)h2, where ni'.n is the 

 gametic ratio for coupling or repulsion. 

 The resulting numerical ratios for coupl- 

 ing (Table I) show a deficit in reces- 

 sives increasing as the coupling be- 

 comes closer, and a small excess in 

 homozygous dominants, the ratio of 

 dominants to recessives increasing from 

 over 3:1 to an indefinite extent as the 

 gametic ratio increases. For repulsion, 

 on the other hand, there is a correspond- 

 ingly large decrease in homozygous 

 dominants and a small increase in 

 recessives, the ratio of dominants to 

 recessives varying with the gametic 

 ratio from under 3 : 1 to a limit of 2 : 1 . 

 Hence, wherever there are lethal factor 



combinations, as in species crosses, we 

 may expect to meet with ratios of 

 dominants to recessives in the second 

 generation differing from the usual 

 3 : 1 ratio, because of linkage and cross- 

 ing-over. I have shown (7) that cross- 

 ing-over occurs in some species crosses. 



Tarle I 



Ratios in progeny of AaBh ulant, wlicre n^ is 

 lethal, and A and B are linked. 



Gametic ratios 

 AB or ah: 

 A h or (I B 



Bo-Br.h,^ 



Dom. 

 Rec. 



In the case of Oenothera lamarckiana, 

 the death of half the embryos, as well 

 as the results of crossing, are met by 

 Rcnner's hypothesis; that, after selfing, 

 a pair of factors, or combinations of 

 factors, are lethal to the zygote, when 

 homozygous; so that L2 and h are 

 eliminated, and only Ll embrvos left. 

 Then a factor B, linked with L. will be 

 found in the proportions of mnl>>: {ni^-{- 

 n'^)Bb:mnb2, the numerical results being 

 given in Table II. 



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