Dominant piebald spotting

Piebald spotting is a rare dominant human phenotype. Although the phenotype is encountered sporadically in all races, the patterns show up best in those with dark skin. (a) The photographs show front and back views of affected persons IV-1, IV-3, III-5, III-8, and III-9 from (b) the family pedigree. Notice the variation in expression of the piebald gene among family members. The patterns are believed to be caused by the dominant allele interfering with the migration of melanocytes (melanin-producing cells) from the dorsal to the ventral surface in the course of development. The white forehead blaze is particularly characteristic and is often accompanied by a white forelock h the hair.
Piebaldismisnot a form of albinism; the cells in the light patches have the genetic potential to make melanin, bur, becausethey arenot melanocytes, they are not developmentally programmed to do so. In true albinism, the cells lack the potential to make melanin. (Piebaldism is caused by mutations in c-kit, a type of gene called a proto-oncogene; see Chapter 16.)

[Photos (a) and data (b) from I. Winship, K. Young, R. Martell, R. Ramesar, D. Curtis, and P. Beighton, “Piebaldism: An Autonomous Autosomal Dominant Entity,” Clin. Genet. 39, 1991, 330. © Reproduced with permission of John Wiley & Sons, Inc.]