Genetic linkage between X-linked retinitis pigmentosa and DNA probe DXS7 (L1.28): further linkage data, heterogeneity testing, and risk estimation.

Further linkage data relating X-linked retinitis pigmentosa and DNA probe DXS7 (L1.28) is presented in this paper. The current mean estimate of the recombination fraction (theta) including this and all published data, is 0.09, with confidence limits 0.04 to 0.17 (maximum Lod score of 14.01 at a theta of 0.08). There is no evidence for heterogeneity of recombination fraction between the 13 families for which data are available. However, it is argued that heterogeneity should be assumed to exist for the purposes of risk estimation. Mean estimates and variances of risk are calculated for hypothetical families each with different linkage data. In families in which no recombination has been observed, the mean and variance of risk are sufficiently small for the clinical use of this probe to be acceptable to many.