The molars of the tabby mouse, and a test of the 'single-active X-chromosome' hypothesis.

The sex-linked gene for tabby (Ta) in the mouse, in addition to its effects on fur and skin, has a characteristic dental syndrome (Griineberg, 1965). Some tabby heterozygotes have a spectacular mixture of molar teeth, some of which are normal, others which are tabby, and some which combine features of both. This prima facie evidence in favour of the single-active X-chromosome hypothesis of Lyon (1961) can be turned into a more exacting quantitative test of that concept. An autosomal mimic of tabby, called crinkled (cr; linkage group 14), with the same dental syndrome, offers opportunities for further tests of the hypothesis. It will be seen that, contrary to first impressions, the quantitative study reveals a situation which is not in agreement with the Lyon hypothesis.