An abnormal small acrocentric chromosome with enlarged short arm was found in a mother, grandfather and child, the latter with Down’s syndrome. It is probable that the chromosome originated by a mutational event in the sex cells of one of the great-grandparents as it was present in only one out of their seven children. Data are given on blood and serum types and the linkage information they provide. The nature and significance of the chromosome is discussed and it is concluded that the most likely explanation of the abnormality is heritable alteration of coiling and condensatio