s\vi.m;li;. — vaiu.vïio.n in FIKSÏ CKNEK.VTIO.N UYISKIIIS. 5.S1 



VARIATION IN FIRST GENERATION HYBRIDS IMPERFECT DOMINANCE) : 

 ITS POSSIBLE EXPLANATION THROUGH ZYGOTAXIS' 



l!y WalterT. SWINGLE 



U. s. Uc|iaiimenl o{ .ngrii;ul(uie, Wasliiiijjlon, 1). C. 



CuiTcnl Ihcories ol' lici-cilily ami vai'ialic^ii t;i\e no mcllioil ol' rxplaiiiinij;' 

 variaLions iii (irsi génération livljiicls Ijelween pure Ijrcd parents. 



Il is generally assumcd lliat ilill'erences in tlie cliaracters of organisms are 

 due lo corresponding ditlerences in liie eomposilion of the gamètes that unité 

 lo form tlic individual organism. No douLt that is Irue In many cases, but itis 

 my purpose to point ont cases in which tliis assumption cannol be suslained. 



Sonie first génération hybrids show a very considérable range of variation, 

 far more than can possibly be accounted for by any quablativedifTerences in llie 

 chromosomes or other bearers of heredity received from the two parents. 



Inasmuch as during the whole of the first (or conjugale) génération Ihc 

 chromosomes from tlie hvo gamètes persisl side by side in an unfused stale, 

 such variations cannol be due lo any quantitative différences in Ihe bearers of 

 lieredily possessed by Ihe sister hybrids. 



If proof can be given lo show that in certain speiilic cases pairs of gamètes 

 of identical hereditary composition give rise lo very diverse organisms, the ^\ay 

 lias been opened for a gênerai reinvestigalion of tiie validity of our modem 

 théories of heredity. 



Some years ago wlicn 1 carried un iireediiig work on iiiaize iu conjunclion 

 with my leacher, Ihe laie Prof. W. A. Kellerman, I was profoundly impressed 

 by the diversity shown in dilTerenl plants ot first génération hybrids of identical 

 pure parentage. AH the ears on a single plant, no maller how many, were of 

 Ihe same gênerai type, but Ihose on other jilants descended from the same hvo 

 races were différent, often very decidedly dillerenl. A consideralile number of 

 hybrids showed Ihis behavior-*. 



Il musl be remembered Ihal such hybrids are made up of cells, Ihe nuclei 

 of which contain side by side the identical chromosomes derived from the Iwo 

 parental gamètes. If Ihe parents are pure bred, ail the first génération hybrids 

 are derived from identical ijameles and havc an idenhcal equi pmcni of rliromo- 

 aames''. 



Any difl'erences observed between such hyl)rids uiust be due lo dillerences 

 in the expression of hereditary lendencies and not due to différences in llieir 

 transmission lo the gamètes as Cook bas pointed oui'. 



In récent years I bave found slriking diirerences between first génération 

 sister hybrids of Uvo very dilTerenl species of Cilrus, which although showing 



1. OuuiiiiuiiicMlidii r^iilij à la troisième sûance de la ConfL'i'euce. 



'2. Kelk'i'iiKiii, W. A., aiid Swingle, W. T., 1890. Crossed Corn tlic Seeuud Vear. iji '2nd Animal 

 Reporl Kan. Agric. Exp. Sta. pp. 334-346, pi. 11. Tojieka. 



3. Cook, 0. F., and Swingle, W. T., 1903. Evolution uf Ccliular .Slruclinvs. Ijnll. 81, !i.P.I.,U.S. 

 Dept. Agriculture. (Aug. 4). 



4. Cook, 0. F., 1907. Transmission Inlieritance dislincl IVom Expression Iiiheritance in Science 

 (N.S. 25: 911-912 n. (ji9, 7 June). 



