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Becker's allelic model to explain unusual pedigrees with spinal muscular atrophy

Sirs, Bouwsma & Leschot (1986) described seven unusual pedigrees with spinal muscular atrophy (SMA) and discussed Becker’s allelic model (Becker 1964) as one possible explanation. Since our first published pedigree (Zerres & Grimm, 1983) we observed 5 more families with an unusual genetic pattern, not consistent with simple modes of inheritance (Fig. 1 D-H). Becker’s allelic model could not only be one possible explanation in these pedigrees but also in families with siblings with different types of SMA (Fig. 1 A-C). Furthermore Becker’s model could explain recurrence risks for siblings as well as for children of patients with SMA type Kugelberg-Welander without postulation of different modes of inheritance. Heterogeneity on the other hand is not ruled out in all cases. As part of our cooperative study* on the inheritance of SMA intended to calculate gene frequencies, Becker’s model will be