Evidence from Mutable Genes Concerning the Origin of the Germ Line
In recent years, studies on aggregation and injection chimaeras and on the genetic mosaics that result from X-chromosomal inactivation have done a great deal to elucidate what goes on in the early stages of mammalian development. Other kinds of genetic mosaic, namely those arising through forward and reverse mutation at particular loci, can also provide useful information. This is particularly so because the initial mutational event in these mosaics can occur at different stages in development, from the first cleavage division on, and can therefore provide us with a sort of running commentary on developmental processes; whereas the initiating events with respect to X-chromosome inactivation and the formation of chimeras are limited to very early stages in embryogenesis. Another point which may be of importance is that there would seem to be much less chance of preferential proliferation of one of the clonal components of such a mosaic than in most chimeras. In these mosaics, the genetic difference between the clones normally involves two viable alleles at a single locus controlling some aspect of coat colour. In chimeras, however, there are usually many genetic differences between the components.
KeywordsGerm Line Coat Colour Primordial Germ Cell Reversion Frequency Embryonic Life
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