Simple methods have been proposed as screening tests for major loci. These methods rely primarily upon the detection of differences in within sibship variances expected for segregating and non-segregating sibships when a major locus is present. In the present study, computer simulation was used to investigate power and robustness of the test statistics. Power of the analyses depends upon the specific major locus model, but, in general, their application is quite practical for small samples. The test statistics were shown to be sensitive to deviations from normality, but robust under the conditions of either a polygenic or environmental model. Application of the test procedures to sibship data from the Boulder Family Study led to significant results for a three-dimensional spatial rotation test, but results for height and weight were non-significant. The simplest interpretation of the results for the spatial performance test was in terms of a sex-linked major locus.