1.8 The zebrafish, a model genetic organism, has been instrumental in helping to identify genes encoding pigmentation differences among humans. (a) Human ethnic groups differ in degree of skin pigmentation. (b) The zebrafish golden mutation is caused by a gene that controls the amount of melanin in melanosomes.

[Part a: Barbara Penoyar/Getty Images; Amos Morgan/Getty Images; Stockbyte/Getty Images. Part b: Keith Cheng/Jake Gittlen, Cancer Research Foundation Penn State College of Medicine.]