We consider the problem of detecting and estimating the strength of association between a trait of interest and alleles or haplotypes in a small genomic region (e.g. a gene or a gene complex), when no direct information on that region is available but the values of neighbouring DNA-markers are at hand. We argue that the effects of the non-observable haplotypes of the genomic regions can and should be represented by factors representing disjoint groups of marker-alleles. A theoretical argument based on a hypothetical phylogenetic tree supports this general claim. The techniques described allow to identify and to infer the number of detectable haplotypes in the genomic region that are associated with a trait. The methods proposed use an exhaustive combinatorial search coupled with the maximization of a version of the likelihood function penalized for the number of parameters. This procedure can easily be implemented with standard statistical methods for a moderate number of marker-alleles.