A strategy to reveal high-frequency RFLPs along the human X chromosome.

Fifteen human X-chromosome-specific DNA fragments, localized to particular regions of that chromosome, were used to search for restriction fragment length polymorphisms. A screening panel prepared by digesting DNA from only two females and one male with 24 restriction enzymes was sufficient to reveal two-allele polymorphisms among one-third of the probes tested. These polymorphisms, as theoretically anticipated, showed minor allele frequencies above 20%, as a rule. Such high-frequency polymorphism allowed identifying females, from pedigrees segregating three X-linked diseases, who were multiply heterozygous for polymorphic loci spread throughout the X chromosome. In addition, two of the 24 enzymes tested with these X-specific probes, Msp I and Taq I, generate fragment sizes in DNA-blotting experiments that, on average, are significantly larger than expected from nearest neighbor predicted recognition site frequencies.