Every physician who must make decisions in the management of a child with Down's syndrome faces the problem of whether or not chromosomal analyses should be done and, if so, upon which members of the family. The purpose of such analyses is the identification of those individuals who may be expected to have an increased incidence of mongolism among their offspring as a result of a translocation of a 21 chromosome, a 21 isochromosome, mosaicism for 21-trisomy, or 21-trisomy itself. However, the high cost and difficulty of obtaining chromosomal analyses require that their number be kept to the minimum necessary, and that appropriate short cuts in the laboratory method also be applied whenever possible. The following paragraphs will attempt to lay a base for, and to outline, a logical and practical approach to the chromosomal screening of relatives of patients with Down's syndrome.
Table I lists various parental abnormalities involving the 21 chromosome, together with the theoretical proportion of cases of mongolism to total live births for each. In considering this table, the reader should keep in mind that it refers to an estimated 1 or 2% only of parents of patients with Down's syndrome. In an estimated 98-99% of instances, both parents are chromosomaliy normal.
Although the observed proportion of cases of mongolism to total live births approximates the theoretical ratio among the infants of mothers with the phenotype characteristic of Down's syndrome, the observed proportion is lower than the theoretical ratio among the children of parents with a normal phenotype and a chromosomal abnormality predisposing to mongoloid offspring.
Multiple Chromosomal Aberrations in a Patient with Acute Granulocytic Leukemia Associated with Down's Syndrome and Twinning