There is a proposed method for evaluation of the minimal sample size needed for the predefined quality of the linkage decision. It is applied to samples of arbitrary structured pedigrees ascertained via proband in accordance with different ascertainment schemes. The quality of the linkage decision is determined by the I and II type errors of the likelihood ratio constructed with a simple alternative hypothesis about the recombination fraction value. Comparison of these minimal sample sizes for different ascertainment schemes leads to a possibility to choose the best of them when the model of genetic control of two traits is given. A possibility to evaluate each of these ascertainments by a cost of the sample collection and to point out the cheapest is considered.