Direct identity-by-descent mapping is a technique for narrowing down the location of the gene
or genes responsible for a given genetic disease to small segments of the genome. The technique
involves DNA comparisons between pairs of affected individuals. The data generated are in the
form of matching segments of the genome, representing regions likely to be identical-by-descent
(IBD). Regions in the genome over which there are significantly more segments aligned than is
expected by chance are taken as candidate regions for the disease gene or genes. Due to the complex
geometric nature of the data, significance testing involves certain mathematical difficulties. We
present here a new method for measuring this significance. This method introduces a novel statistic
and is appropriate whether or not the relationships between the paired individuals are known. We
give examples that we have calculated by implementing this method, including an application to
real data.