Birth Defects

Abstract

Seventy percent of all known birth defects involve the head and neck. Of these, 10% result from chromosomal abnormalities, 20% are single gene defects producing complex syndromes, while the remaining 70% are polygenic or multifactorial. Clinical geneticists recognize four classes of birth defects, summarized by Spranger et al. (1982) as:

• • malformations—inherited defective developmental processes;

• • deformations—mechanical disruptions to embryos;

• • disruptions—external interference with developmental processes, as when a teratogen is administered; and

• • dysplasias—neoplastic changes such as those discussed in the previous chapter.