Restriction site polymorphisms are normal inherited variations in DNA that can be readily detected by restriction endonuclease analysis. Currently, 17 such polymorphisms are recognized within a 60 kb (kilobase) stretch of DNA which includes the β-globin gene complex. Because of their proximity to the β-globin gene, often these restriction site polymorphisms can be used to predict inheritance of β-globin variants that produce disease. For example. restriction site polymorphisms can be used for prenatal diagnosis for the large majority of couples at risk of having a child with β-thalassemia. When each member of such a couple is heterozygous at one or more of these 17 sites, family studies are usually successful in determining which forms of the polymorphism are co-inherited with the β-thalassemia genes in that particular family. Subsequently, study of fetal DNA isolated from amniocytes obtained by midtrimester amniocentesis or from chorionic villi obtained by first trimester chorion biopsy will reveal which DNA polymorphisms that fetus has inherited. By deductive reasoning one can then predict which β-globin gene it has co-inherited. Because of the general nature of these polymorphisms, which are related to the β-globin gene and its variants only because of their proximity on chromosome 11, they are potentially useful in the prenatal diagnosis of any β-chain hemoglobinopathy. Some hemoglobinopathies (including α-thalassemia, sickle cell anemia, and some cases of β-thalassemia) can be detected directly by DNA analysis. In these cases in utero diagnosis does not need to rely on restriction site polymorphisms, which require preliminary family studies and are not applicable in all at risk pregnancies. Recently, genetic probes, which are necessary for detecting restriction site polymorphisms, have been isolated for sequences of several genes whose protein products are important in blood coagulation. These include probes for all three genes whose polypeptide products combine to form the fibrinogen molecule as well as probes for the prothrombin, Factor IX, Factor VIII, and antithrombin III genes. Defects in these genes are expected to be the causes of afibrinogenemia, prothrombin deficiency, hemophilia B, hemophilia A, and antithrombin III deficiency, respectively. From experience with other genes, it is expected that restriction site polymorphisms within and/or flanking these genes will be found. If the defects which produce the above disorders cannot be directly detected or if the disorders are found to be very heterogeneous, restriction site polymorphisms may be quite useful in the prenatal diagnosis of some of these disorders of blood coagulation.
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