The emerging genetic theories of unstable DNA, uniparental disomy, and imprinting.
Usually, genetic diseases segregate in a stable, predictable fashion throughout a family. For many inherited disorders, a mutation at the DNA level has been characterized, and prenatal diagnosis is available. Recently, however, the dogma of traditional mendelian inheritance has been challenged by the discovery of new patterns of gene segregation and expression. Unstable DNA segments varying in size have been associated with both fragile X syndrome and myotonic dystrophy and can now be diagnosed by direct DNA analysis. Uniparental disomy, the inheritance of both members of a chromosome pair from one parent, has been documented in autosomal and X-linked recessive diseases. Imprinted genes, the expression of which is dependent on the parent of origin, are being increasingly identified, especially in syndromes with abnormalities of growth. In the near future, the number of prenatally diagnosed disorders will expand as tests for unstable DNA, uniparental disomy, and imprinting become widely available.