PEDIATRICS Vol. 35 No. 3 March 1965, pp. 463-469
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THE ROLE OF CHROMOSOME TRANSLOCATION IN THE RECURRENCE RISK OF DOWN'S SYNDROME

C. David Petersen B.A.1 and Luigi Luzzatti M.D.2

1 Department of Pediatrics, Stanford University School of Medicine Palo Alto, California
2 Department of Preventive Medicine, Stanford University School of Medicine Palo Alto, California

Increasing knowledge of the chromosomal pattern of patients with Down's syndrome has helped in elucidating the possible hereditary factors present in this condition. The pattern of transmission of the translocation type from a parental carrier to the offspring is fairly well defined. There is still a question concerning possible hereditary factors in the transmission of the more usual type, characterized by simple trisomy of chromosome No. 21. While the over-all incidence of Down's syndrome increases with maternal age, the chance for a second affected child is greater in the younger maternal age group.

The present study has been undertaken to evaluate: (1) the frequency of inherited translocation among affected children born to younger mothers, and (2) the significance of translocation in the increased recurrence risk of the young mother. A total of 227 index patients with Down's syndrome, born to mothers age 15 to 29 years, was collected. Of these, 42 were studied in our laboratory and the others collected from the literature. In this group, the incidence of the usual type of trisomy 21 was 92.1% and the incidence of the translocation type 7.9%. In 1.8% of the patients the mother carried a D/21 translocation. On the basis of the frequency of maternal D/21 translocation in this group and the expected ratio of affected offspring born in the general population a recurrence risk has been calculated for all mothers with an affected child purely on the chance they may carry a D/21 translocation. This calculated recurrence risk decreases with increasing maternal age and never approaches the figures of the observed total recurrence risk available in the literature. After age 35, data in this study concur with those in the literature, failing to demonstate an increased recurrence risk over the general population.

Though the data are by necessity small, the figures in this study tend to suggest that only a part of the increased recurrence risk of the young mother is due to the possibility she may carry a translocation. There are undoubtedly other still ill-defined factors that may predispose to a recurrence of affected children in these mothers. These factors are discussed but more data are necessary for accurate genetic counseling.

Submitted on May 25, 1964
Accepted on September 25, 1964