Cortical index (CI) is the ratio of the combined cortical thickness to the
total diameter of the bone. It serves for the assessment of the geometric p
roperties of bone and for indirect evaluation of bone mass. CI is a useful
predictor of osteoporosis. The aim of the present study was to test the hyp
othesis of major gene control of CI variation in a large sample of pedigree
s from Chuvashia, Russia. Complex segregation analysis revealed that the ma
jor gene model of CI inheritance is the best fitting and most parsimonious
for the present data. Parameters of the genotype-gender specific dependence
of CI variation on age were estimated simultaneously with other parameters
in the segregation analysis. The results of analysis showed that not only
the baseline level of CI but also the age at onset of the involutive bone c
hanges (inflection point) and the rate of the CI decrease with age (slope c
oefficient) are under control of the same major gene. Nonmajor gene effects
shared by pedigree members (residual familial correlations) were found to
be statistically insignificant. Approximately 73% of inter-individual varia
tion in CI was attributable to the effects explicitly included in the model
. (C) 2000 Wiley-Liss, Inc.