The transmission/disequilibrium test (TDT) is a powerful method of locating disease genes. The
TDT was originally proposed for use in studies of qualitative traits in families with both parents
available. Recently, the TDT has been extended to studies of qualitative traits in sibships without
parents available and in families with one parent available. It has also been extended for use in
studies of quantitative traits in families with both parents available and in sibships with multiple
offspring. In this paper, we first propose a new class of TDT-type tests for linkage in the presence
of linkage disequilibrium for use in studies of families with both parents available. The TDT of
Spielman et al. (1993) for qualitative traits and the TDT of Rabinowitz (1997) for quantitative traits
are special cases of the new tests. Second, we propose a new class of TDT-type tests for linkage for
use in studies of families with one parent available. Third, we study the validity and the power of
the tests using simulations. Finally, we propose a method of combining data from different types of
families. The combined test is valuable and allows researchers full use of the available data in
detecting linkage between a marker locus and an unobservable quantitative trait locus. An
important feature of the tests proposed in this paper is that no assumptions on the distribution of
the quantitative traits are needed.