Genetic anticipation and musculoskeletal disease.

In 1985, Strom et al described two families in which rheumatoid arthritis (RA) appeared in several members over three consecutive generations.' Although these pedigrees taken alone could implicate dominant inheritance, the overall picture with RA pedigrees indicates a complex mode of inheritance, with incomplete penetrance of predisposing genes and probably several genes being involved in the diseases process. In addition, an interesting observation in both pedigrees described by Strom's group and one which is difficult to understand in terms of classical Mendelian genetics is that the disease tended to