title: Trisomy 21: rate in second-degree relatives.
abstract: The risk of recurrence for trisomy 21 offspring in couples with one affected child is accepted to be 1%. The rate of trisomy 21 in second- and third-degree relatives is not established. Tamaren et al [Am J Med Genet 15:393-403, 1983] reported a Down syndrome rate of 0.67% among aunts and uncles (second-degree relatives) of a Down syndrome proband. This eightfold rate increase over that for a control group can also be applied to nieces and nephews who are also second-degree relatives. One routinely discusses amniocentesis for lower risk levels; thus, should one offer amniocentesis to every sib of a trisomy 21 proband? We obtained extensive family histories on 141 trisomy 21 kinships to ascertain the rate of trisomy 21 in second-degree relatives. The rate for sibs agreed with the accepted 1% rate. Among aunts and uncles we found no trisomy 21 individuals. We conclude that the rate of trisomy 21 in second-degree relatives does not appear to be sufficiently increased to warrant routine discussion of amniocentesis with sibs of trisomy 21 individuals.
mesh_terms: D000328:Adult; D004314:Down Syndrome / Q000235:genetics*; D005260:Female; D005817:Genetic Counseling; D006801:Humans; D008297:Male; D008423:Maternal Age; D012306:Risk; D011795:Surveys and Questionnaires