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.


Retinoic Acid Neural Tube Neural Crest Neural Crest Cell Avian Embryo 


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Copyright information

© Springer Science+Business Media New York 1999

Authors and Affiliations

  • Brian K. Hall
    • 1
  1. 1.Department of BiologyDalhousie UniversityHalifaxCanada

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