The distribution of melanocytes in the dorsal coats of a series of chimaeric mice.

Chimaeras, made by aggregating two embryos from the randomly-bred Q-strain, and X-inactivation mosaics both have similar, balanced distributions of pigmentation in the dorsal coat, while (C57BL X C3H) F1 reversible Recessive chimaeras tend to have an unbalanced pigment distribution, with a higher proportion of F1 pigmentation in the posterior than in the anterior part of the coat. This observation can be explained if differences in the timing of the migration of the two melanocyte populations from the neural crest cause anterior-posterior differences in selection pressures.