Figure 2-19 Piebaldspotting, a rare dominanthuman phenotype.Although the phenotype isencountered sporadically inall races, the patterns showup best in those with darkskin. (a) The photographsshow front and back viewsof affected persons IV-1,IV-3, III-5, III-8, and III-9from (b) the family pedigree.Notice the variationbetween family members in expression of the piebald gene. It is believed that the patterns are caused by the dominant allele interfering with the migration ofmelanocytes (melanin-producing cells) from the dorsal to the ventral surface in the course of development. The whiteforehead blaze is particularly characteristic and is oftenaccompanied by a white forelock in the hair.Piebaldism is not a form of albinism; the cells in the light patches have the genetic potential to make melanin, but because they are not melanocytes, they are notdevelopmentally programmed to do so. In true albinism, the cells lack the potential to make melanin. (Piebaldism is caused by mutations in c-kit
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