Notes to Chapter 7

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1In an analysis of the position effect variegation in female mice heterozygous for the flecked X-autosome translocation (see Chapter 3, Section II, H), Cattanach ( 1974) provides data which are in broad agreement with the "standard" pattern as derived by Mintz from allophenics. He too concludes that three bands can be allotted to the head (and neck) but that perhaps seven, rather than six, occur on the body, four in the thoracic and lumbar regions, and probably three more in the rump area. The uncertainty about the exact number stems from the great amount of cell mingling in this latter area. In agreement with Schaible ( 1969; see Chapter 9, Section I), Cattanach also draws attention to a twin head-spot. Taken together, then, according to his reckoning, including the heat spot, there are at least 10 and more probably 11 pairs of sites on the head and body which are colonized by single clones of cells.

2It is interesting to note that the occurrence of bands in the head region provides concrete evidence for the long held theory that mammalian embryos might have anatomically "invisible" head somites ( Mintz, 1970).

3Since in fz/fz <--> Fz/Fz allophenic mice some hairs, especially at the clonal boundaries, are obviously more morphologically intermediate between abnormal fz/fz hairs and normal Fz/Fz hairs, and since in A/A <--> a/a allophenics some hairs with yellow pigment display less than the usual amount of yellow banding characteristic of agouti hairs ( Mintz and Silvers, 1970; Mintz, 1974), the dermal component of a single hair follicle apparently must be derived from at least two cells ( Mintz, 1974; see also McLaren and Bowman, 1969).

4Some of this criticism ( Lyon, 1968) was aimed at Mintz's original assertion that the two color types of primordial melanoblasts were not arranged randomly ( Mintz, 1967), an assertion which Mintz now agrees is not the case ( Mintz, 1970). Indeed, in a more recent paper in which Dr. Lyon is a coauthor ( Cattanach et al., 1972) she apparently accepts Mintz's interpretation of the archetypal pattern.

5It seems most likely that if, e.g., Miwh affected only melanoblast differentiation and not melanoblast survival, a situation which would result in the continued occurrence of melanoblasts in the skin (and elsewhere), that the patterns displayed by +/+ <--> Miwh/Miwh allophenics would more closely resemble the banded patterns which exemplify +/+ <--> c/c allophenics. This is especially the case since Mayer ( 1970) observed that the prior presence of albino melanoblasts in the skin prevented the entrance of normal (+/+) melanoblasts (see Chapter 11, note 11).

6Although the coat color patterns of allophenics do not necessarily reflect the relative proportions of each strain in the other organs (see Mintz and Palm, 1969; Mintz, 1974; Gordon, 1976), there are situations where such a correlation has been found. Employing the combination B10CBAF1 ( A/a;C/C) <--> SJL ( A/A;c/c), Mullen and Whitten ( 1971) observed that the allophenics which produced only SJL sperm had the greatest proportion of white in the coat (42% average); those which produced only B10CBAF1 sperm had the lowest (12%); and those which produced both sperm were intermediate (23% white coats). They also observed that whereas normal sex ratios occurred in multicolored allophenics in which one genotype predominated, in those in which neither component predominated, the sex ratio approached 3 male : 1 female. R. Roberts and his colleagues ( 1976) have made allophenics from mice selected for large (L) and small (S) body size, marked with different coat colors. Their observation that the growth of these allophenics was related to the proportion of L pigmentation in the coat indicates a correlation between the proportion of melanocytes of L and S origin and the proportions of L and S cells in whatever tissue (or tissues) regulate growth. Finally, Markert and Petters have produced some hexaparental mice, i.e., animals derived from aggregates of three genetically marked 8-cell embryos, and one of these, as well as involving the aggregation of four genetically marked embryos, displayed three of the markers (albinism, black, and recessive yellow). These tricolored mice, which exhibited patterns in accord with the supposition that they had been derived from clonal initiator cells arranged linearly from head to tail, are important because they demonstrate that, in the 64-celled blastocyst, at least three cells, and probably only three, are the source of all adult tissues ( Mintz, 1970; Markert and Petters, 1978).

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