New methods of segregation analysis of alternative traits have been developed. These methods make it possible to take into account the sex and age specificity of the disease manifestation. Hence, they extend the range of genetic hypotheses to be tested and ensure the correct analysis of inheritance of complex pathologies in humans. Segregation analysis of idiopathic scoliosis performed in this study demonstrates the possibilities of the new methods. Based on pedigrees of 93 probands, it has been demonstrated for the first time that the inheritance of severe (degrees II to IV) forms of this disease can be described by a model that assumes a dominant major gene with incomplete, sex- and age-dependent penetrances of all genotypes. According to this model, severe forms of idiopathic scoliosis do not develop if the mutant allele is absent (the penetrance of genotype A1A1 is zero). The probabilities of the disease for subjects with genotypes A1A2 and A2A2 are similar and approximately equal to 0.3 and 0.5 for males and females, respectively. Mild (degree I) forms of idiopathic scoliosis are heterogeneous. A progressive disease may be expected only in the patients that carry the mutant allele.