Multiple Xbal polymorphisms for carrier detection and prenatal diagnosis of haemophilia A

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Abstract
Three Xbal restriction fragment length polymorphisms (RFLPs) can be detected using the factor VIII‐intron 22 probe (p482.6) in a Xbal‐KpnI double digest of genomic DNA. The Xbal (A) site had been reported by Wion et al (1986) to be in intron 22. while the two additional sites. Xbal (B) and Xbal (C), are shown here to be X‐linked and close to the Xbal (A) site. The frequencies of heterozygosity for these three sites are 0.49. 0.18 and 0.30 respectively. In 75 females the observed heterozygosity rate for the Xbal (A) site is 0.41 and this increased to 0.57 with the two additional sites. Care should be exercised when interpreting the Xbal RFLPs, since the 1.4 kb Xbal/Kpnl fragment and the 4.8 kb Xbal fragment are associated with both positive Xbal (A) and Xbal (B) sites. By the combined use of the multiple Xbal polymorphisms with the Bell site in intron 18, the carrier detection rate would increase to 67%. Four prenatal diagnoses had been performed using the multiple Xbal polymorphisms.