In order to reveal the amount of concealed genetic variation maintained in cultivated buckwheat populations, the frequency of chlorophyll-deficient abnormalities per gamete was estimated by conducting more than 3000 full- sib matings. The following types of abnormalities were observed in the cotyledons; albino (0.37%), yellow (3.56%), pale yellow (4.86%), pale green (7.40%), variegated (2.39%) and morphological (2.04%). Each type was also found in the foliage leaves at about a half the frequency in the cotyledons. The total frequency did not vary so much among the populations, ranging from 18.0 to 34.5%, with the exception of the Togakushi population, which had a low frequency. A significant north-south cline in the frequency was observed in fall type buckwheats. The frequency of abnormalities was also examined in randomly mating populations in farmer′s field and the total frequency was 0.020-0.072% in the cotyledons and 0.016-0.059% in the foliage leaves. The low frequency of homozygotes in randomly mating populations may be due to a large population size, panmixia by insect pollinations, and the virtual absence of inbreeding. Based on the estimated frequency per gamete and the estimated frequency in randomly mating populations, the number of loci affecting chlorophyll-deficiencies and the deleterious allele frequency per locus were estimated. The approximate number of loci was at least 70, probably close to 100, and the allele frequency was around 0.001-0.004, implying that the detrimental genes in buckwheat populations are maintained by mutation-selection balance.