The causal mutation of a harmful defect may be already known (as is the case of CWC15, or of the Weaver syndrome [10]) or not yet (for example the mutation behind syndactyly in Holsteins [11]): in this latter case, haplotypes associated with the defect can be detected [12, 13] (e.g. the HHM haplotype associated to syndactyly). This evidence concerns the gene CWC15 and syndactyly.