Chapter 7

The Pigment Patterns of Allophenic Mice and Their Significance

II. Modified Patterns

The variability which is observed in the standard allophenic pattern can be readily explained simply by assuming different total proportions of clones of the two genotypes and the permutations and combinations of how the progenitor cells are arranged on the mid-dorsal line. Thus, if by chance in the example we have chosen, three b/b stem cells occurred in succession, the resulting brown band would be three times as wide (and it should be noted its origin would be inferable only from a knowledge of the standard pattern). Also contributing to pattern variability is clonal selection upon which factors such as differences in cell viability and differences in rates of clonal expansion may be superimposed. Closure of skin areas and regional disparities in growth rates, etc., also contribute to modifying the pattern. For example, in some allophenic mice the entire complement of clones of one of the colors produces narrower bands relative to the other color, and this almost certainly indicates that the melanoblasts of one type have a greater proliferating capacity and therefore can encroach upon the territory of neighboring clones derived from cells of the other genotype ( Mintz, 1970). Indeed, there are so many factors which can contribute to varying the standard pattern that it is not surprising that it is observed relatively rarely.