The present invention relates to a method for determining the genotypes of polymorphic loci amplified in a mixture of restriction fragments, using an oligonucleotide sequence that is essentially complementary to part of the target restriction fragment, and located adjacent (upstream) to the polymorphism to be detected. The method involves contacting the mixture of restriction fragments with the oligonucleotide sequence under hybridization conditions, such that when the target restriction fragment is present, a hybrid is formed between the target restriction fragment and the oligonucleotide sequence. When the resulting hybrid has at least one unpaired nucleotide of the target restriction fragment and the oligonucleotide being present, and the at least one labeled nucleotide or nucleotide analog is complementary to the at least one unpaired nucleotide of the target restriction fragment directly adjacent to the 3' end of the oligonucleotide sequence, the nucleotide sequence is extended with a labeled nucleotide or nucleotide analog, which can be detected.