Investigating the numerical effects of ascertainment bias in linkage analysis: Development of methods and preliminary results

It is general practice to have nonsingle ascertainment of pedigrees for linkage studies, along with intrafamilial sampling that is dependent on who among the related individuals was initially ascertained (Proband dependent or PD sampling). Vieland and Hedge [1995; 1996] have shown that under these conditions, the likelihood used in calculating the led score is not strictly correct and can produce asymptotically biased estimates of the recombination fraction, Θ. However they speculated that this bias would be small in most applications. This paper presents preliminary work aimed at quantifying the numerical magnitude of the bias introduced by PD sampling and nonsingle ascertainment in linkage analysis. We considered five generating models where we varied the ascertainment procedure, intrafamilial sampling scheme, and the sample size for each model. In this limited initial set of simulations, asymptotic bias in Θ appears to be trivial, while PD sampling procedures can increase the efficiency of Θ. These preliminary results support the view that the advantages of unsystematic ascertainment may offset any small estimation bias that may arise.