The use of polymorphic microsatellite markers for mapping alternatively distributed behavioral traits with incomplete penetrance was studied with special reference to inheritance of predisposition to catalepsy in mice. Using only backcrosses as a segregating population, the major gene for catalepsy was with high likelihood mapped to region 30-75 cM of murine chromosome 13. It was also established that this gene determines 20% of the trait penetrance whereas the remaining 31% of the observed penetrance are accounted for by numerous polygenes.