Parent to child transmission of the thrombocytopenia absent radius (TAR) syndrome.

We report on cases of the thrombocytopenia absent radius syndrome (TAR) in a family with the first documented occurrence of parent-to-child transmission. At least three other families have been reported in which TAR has been transmitted across generations. The pattern of transmission in these cases is not consistent with the simple autosomal recessive mode of inheritance which has been proposed. TAR syndrome may be a genetically heterogeneous disorder of the result of one of a group of related alleles. Given the increasing evidence for genetic and causal heterogeneity in TAR together with its similarity to conditions such as Holt-Oram, WT Limb-Blood and SC-Roberts Phocomelia syndromes it may be reasonable to view TAR as a developmental field defect rather than a genetic syndrome. Genetic counseling of affected individuals and their families should be modified to reflect the possibility of a recurrence risk as high as 50%.