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 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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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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