Segregation analysis using a model with age and gender effects was applied to 
101 pedigrees ascertained through a proband with idiopathic scoliosis. The 
transmission probability model was used to detect major gene effect. When we 
analyzed the pedigrees where affected status was assigned to persons with a 
Cobb's angle of more than 5 degrees we did not detect a significant major gene 
effect. However, when the affected status was assigned to persons with 
pronounced forms of disease only (a curve of at least 11 degrees) a significant 
contribution of a major causal gene could be established and inheritance could 
be described according to a dominant major gene diallele model, assuming 
incomplete sex and age dependent penetrance of genotypes. According to this 
model, the pronounced forms of idiopathic scoliosis should never occur in the 
absence of the mutant allele. This indicates that only the carriers of the 
mutant allele develop pronounced forms of the disease. At the same time, only a 
fraction of the carriers of the mutant gene should manifest the disease (30% of 
males and 50% of females).