Risk of Recurrence of Chromosomal Abnormalities
Mark D. Ludman MD1
Fred Gilbert MD1
Kurt Hirschhorn MD1
1 Division of Medical Genetics, Department of Pediatrics, Mount Sinai School of Medicine, New York, NY 10029
Chromosomal abnormalities are an important cause of mental retardation and of congenital malformations in both live and stillborn infants. Loss or addition of chromosomal material has been found in between 5% and 6% of perinatal and pediatric autopsies1 and in approximately 1/250 newborns.2 A pediatrician in the course of his or her professional lifetime is, therefore, likely to be called upon to counsel parents who have had a child or a stillborn infant with a chromosomal abnormality and who wish to know the risk of recurrence of similar problems in future pregnancies.
Down syndrome, the most common chromosomal abnormality seen in liveborn infants (with a frequency of approximately 1/700), will serve as the prototype for the discussion that follows. It is not the aim of this article to describe chromosomal structure or nomenclature in detail or to delineate the physical features and medical complications of different chromosomal disorders. For this the interested reader is referred elsewhere.3 Nevertheless, some familiarity with the mechanisms by which such abnormalities can be produced will help in understanding how estimates of recurrence risks for particular chromosomal abnormalities have been determined. Such mechanisms will, therefore, be discussed when it is felt to be appropriate.
CLASSIFICATION OF CHROMOSOMAL ABNORMALITIES
The normal complement of human chromosomes is 46, 22 pairs of autosomes and two sex chromosomes (XX in females, XY in males).
