Heritable disorders of hair.

Changes in the quality and quantity of hair occur in a large number of hereditary syndromes. Because of their rarity and our lack of knowledge about their pathophysiology, no entirely satisfactory classification of these hair disorders exists. This review arbitrarily groups the heritable disorders of the hair into five classes: primary hair shaft disorders, alopecia without associated defects, ectodermal dysplasias, changes in hair color, and ectopic hair.