Loss of heterozygosity (LOH) is a technique that can only be used to compare two samples from the same individual.  LOH analysis is often used when identifying cancer-causing oncogenes in that one sample consists of (mutant) tumor DNA and the other (control) sample consists of genomic DNA from non-cancerous cells from the same individual.  RFLPs and microsatellite markers provide patterns of DNA polymorphisms, which can be interpreted as residing in a heterozygous region or a homozygous region of the genome.  Provided that all individuals are affected with the same disease resulting from a manifestation of a deletion of a single copy of the same gene, all individuals will contain one region where their control sample is heterozygous but the mutant sample is homozygous - this region will contain the disease gene.