Familial Down's syndrome A cytogenetical and genealogical study of twenty‐two families

Summary The pedigrees and cytology of 22 families with Down's syndrome in more than 1 member are presented. In 10 families the patients were sibs, in 6 families cousins or more closely related, and in 6 families they were more distant relatives. Inherited 13–15/21 translocation was the cause of multiple occurrence of Down's syndrome in 4 families. In the other families the patients were found to have the standard trisomy 21 karyotype. No isolated translocations were observed and no other translocation type was found. In one family a ‘marker’D chromosome and in another family a small Y chromosome were observed. One patient showed possible mosaicism with trisomy 21 and a few normal cells.