Lack of an enzyme in this pathway can occur due to a spontaneous and fairly common mutation. When this enzyme is missing or inactive, the pathway is blocked, so no melanin can be made. Such animals are said to be melanistic . The black squirrels found in the streets and parks of Washington DC are a nice example. The melanocytes are the cells in this row which are not stained and which do not contain melanin.
Melanocytes are a stable and long-lived population of cells.
The keratinocytes aren't. The life of a keratinocyte is at best measured in days, but the life span of melanocytes is years, probably in most cases the entire life of the animal. As with other normally non-dividing cells, however, sometimes things go haywire. If melanocytes do start dividing, it's a very serious matter, as they lack the ability to control their division. The uncontrolled growth can become highly invasive and move into surrounding areas.