T h e effects of the mamnlotli gcnc causing a short-day pliotopcriodic response in a mutant lvcrc srudied in 1962 and 1966 in six gcnctic populations (PI, P,, F,, F,, B,, B,) derived from thc cross bet\\.ecn h4ammoth Dclcrest, a short-day mutation of thc fluc-cured tol)acco variety Dclcrest, and Praecos, a day-neutral pipc robacco. T h e data slio~vcd thc cxpected nlonofactorial segregation of day-neutral against short-da!. plants, the latter being rccessivc. Considerable amount of positivc hctcrosis I\.as obserrcd for lcaf width. I n thc inhcritaiicc of da).s-to-flo~\:er, in addition t o thc pair of alleles determining tlic short-day response, onc to nvo gene groups are indicated, operating probably i n an addirirc fashion. T n f o and rhrcc gcnc groups are indicated for lcaf ~v i d t h and leaf length, respectively. Paramctcrs describing additive gcne cffecrs nrcrc important for all thc charactcr tcstcd but nonc of rhc paranleters for dominance and digcnic epistatic cffccts wcrc significant. Eridcntly thc scgrcgating il~ajor gcnc for tlic mammoth cl~aracrer did not cxccssivcly influcncc thc rclationship of gcnc cflccts in thc icmaining gcnotypc as dctcrmined by thc gcnerarion-nican analysis. IntroductionIn tobacco, Nicotial~n tabacu771, a spontaneous mutation occurs from time to time which produces plants that do not flowcr ilndcr norillal growing conditions in thc ficld but gro\il tall and producc an esccssivel\. large nuillber of leavcs. This mutation has been callcd gigantism or mammoth charactcr (Allard, 1919). Allard and Carncr (1940) determined that genetical changes in the gcrnm plasm of such mutants arc responsible for the non-induction of flowcring when plants are csposed to long-day photoperiods and that thesc plants may be induccd to flou cr bv short-day photoperiods, such as in the fall. The dependencv of plants on thc iel~gth of daj. for the induction of flowering is generally calicd photopcriodism. Plants that depcnd on the short days for flower induction arc called short-da!~ plants, tliosc that dcpend on long days long-day plants, and those which flower indepcndentl\r of the length of day indcterminatc or daj -neutral. Normal N. tabac~1771 .tvues are day-neutral.
-1T h c short-day p~loto~driodic responsc in tol)acco, as contrastcd to day-neutral response, is conditioned bj. a singlc gcnc diffcrencc, homozygous reccssivc being the mammoth typc (Allard, 1919;Smith, 1950). Lang (1912) considered rhc possibilitv that the difference betwccn short-dav and day-neutral types of ,\,l;rjTland tbl~acco majr be controlled by nlorc thin one faitor, and 0 l t a and i\ilu&ol;a (1957) repoited a three-gcnc hiffcrencc between short-day and dav-neutral t i -~e s of tobacco..