Once mapping techniques have narrowed the disease gene to a relatively small region, attempts will be made to identify all genes in that region by sophisticated molecular techniques such as exon-trapping.
The genes which are found will be subjected to very intensive scrutiny in an attempt to distinguish mutations which are present in affected subjects.
If there are strong a priori reasons to suspect that a particular gene may influence a trait then this gene may be screened for functional polymorphisms even before there is any mapping data to implicate it.