The linkage relationship between the semidwarf gene (sd-1) and the isozyme locus EstI-2 was elucidated using segregating populations derived from crosses between several semidwarf testers and tall rice varieties. Bimodal distributions for culm length were observed in F-2 populations of three cross combinations, including Shiokari/Shiokari (sd-1), Taichung 65 (A, Pn, Pau)/Taichung 65 (sd-1), and Milyang 23/Kasalath. Taking the valley of the distribution curves as the dividing point, two height classes were apparent with a segregation ratio of 3 tall:1 short, demonstrating this character to be under the control of a single recessive gene. An inheritance study of esterase isozymes, based on isoelectric focusing (IEF), showed that the EstI-2 locus had two active allozymes of monomeric structure and one null form, which were designated ''a'', ''b'', and ''n'', respectively (Eun et al. 1990). Semidwarf testers such as Shiokari (sd-1), Taichung 65 (sd-1) and Milyang 23 have an active allozyme designated as EstI-2(aa), while the tall parents, Shiokari and Taichung 65 (A,Pn,Pau), have the active allozyme, EstI-2(bb) and Kasalath has a null form of the allozyme, EstI-2(nn). By dividing F-2 populations based on EstI-2 allozyme patterns, culm-length distributions exhibited trimodal curves. Most of the short plants had the homozygous EstI-2(aa) pattern of the short parents, most of the tall plants had the homozygous pattern, EstI-2(bb) Or EstI-2(nn), and most of the intermediate plants had the heterozygous EstI-2(ab) or EstI-2(an) banding pattern. Linkage analysis indicated that sd-1 and EstI-2 were tightly linked. These findings were also confirmed by segregation analyses in F-3 progenies. No recombinants among 171 F-3 families from the Shiokari/Shiokari (sd-1) combination, five recombinants among 267 F-3 families from Taichung 65 (A, Pn, Pau)/Taichung 65(sd-1), and only two recombinants out of 237 F-3 families from Milyang 23/Kasalath, were found. The recombination values were 0, 1.87 and 0.8%, respectively.
