Recently, Liang et al. ([2001b] Genet. Epidemiol. 21:105-122) proposed a conditional approach to assess linkage evidence on the target region by incorporating linkage information from an unlinked (reference) region using allele shared 11313 (identity-by-decent) from affected sib pairs. This is carried out by conditioning on the 1131) sharing value at the estimated trait locus of the reference region. Since markers considered are typically non-fully informative, the IBD sharing at each marker needs to be estimated (or imputed). In this report, we propose an alternative approach to deal with the 1131) sharing in the reference region. This new approach makes full use of the observed data without having to categorize the imputed 1131) sharing as needed in Liang et al. ([2001b] Genet. Epidemiol. 21:105-122). We compare these two approaches by simulating data from a variety of two-locus models including heterogeneity, additive and multiplicative with either fully informative markers or non-fully informative markers. The performance of both approaches is quite comparable showing consistent estimates of the trait locus and key genetic parameters.
