Provided in part herein is an improved method for the detection of specific polymorphic alleles in a mixed DNA population. The method comprises enriching the relative percentage of a given polymorphic allele that is exponentially amplifiable by PCR. Provided also are methods for selectively enriching target nucleic acid, for example, fetal nucleic acid in a maternal sample. In the case of detecting fetal nucleic acid in a maternal sample, a restriction enzyme is introduced that can discriminate between the alleles of a polymorphic site. In some embodiments, the maternal allele is digested and nucleic acid comprising the paternal allele is relatively enriched.