Sensitive methods for the quantification of DNA fragments can be used to study an individual’s genetic constitution for duplicated regions of the genome or to determine the relative proportion of different DNA fragments in heterogeneous samples such as pooled DNA from different individuals or in samples in which a fraction of the chromosomes carry a mutation. Here, we describe how we are using Pyrosequencing� for this purpose. In Subheading 3. , we describe a sensitive method that can be used to quantify the relative proportion of X- and Y-carrying sperm after sperm sorting in cattle. We also discuss our method for determining the copy numbers at a duplicated locus. This method has been applied to study genetic variation at the KIT locus in pigs, which have a major effect on coat color variation in this species.