In the dorsal epidermis of both C57BL/10J and C3WHe mice, the number of melanocytes positive to the dopa reaction increased from birth to 3 or 4 days of age, and then gradually decreased and disappeared by 30 days of age. However, the number of melanoblasts plus melanocytes positive to the combined dopa-premelanin reaction remained constant until 3 or 4 days of age and then decreased in both strains. Despite the similarity of the change in these populations from birth on, there was a significant difference in the number of such cells between the two strains. Both melanocytes and melanoblasts in C3W He were fewer than in C57BLllCkJ mice.
The F1, Fa, and backcross matings were performed in order to gain some information about the genetic basis of the difference. In the F1 generation the offspring from reciprocal crosses exhibited intermediate values in both populations. The F2 generation included the C3H/He type, F1 type, and C57BL/10J type in a ratio of about 1:2:1 in both populations. Moreover, both reciprocal backcrosses gave ratios of parent type to F1 type, which did not differ significantly from 1:l in either population. These results indicate that the melanocyte and melanoblastmelanocyte populations in the dorsal epidermis of newborn mouse skin are controlled by semidominant genes.