Abstract
In isolated congenital heart disease genetic factors have been shown from family studies, individual pedigree analyses, the frequency of consanguinity, examination of data from twins, and possibly from cytogenetics. In defects of the atrial septum, where data are most complete, genetic factors appear to be important, particularly in secundum atrial septal defect. In the syndromes with abnormal chromosomes in which congenital heart disease is common, the cardiovascular abnormality is probably directly associated with the abnormal genetic material present. In hereditary disorders with normal chromosomes in which congenital heart disease features, the pattern of inheritance suggests transmission by a single gene or group of genes. The cardiovascular abnormality is genetically determined, though it may not become apparent until many years after birth.