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The Genetics of a Tooth Defect in the Mouse
Proceedings of the Royal Society of London. Series B, Biological Sciences
Vol. 138, No. 892 (Sep. 13, 1951), pp. 437-451
Published by: Royal Society
Stable URL: http://www.jstor.org/stable/82751
Page Count: 15
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Absence of third molars occurs in 17.9% of the mice of the CBA pure line, but has not been encountered in the C57 Black strain. In reciprocal crosses between these lines, the character was recessive in F1, but it was not recovered at all in 402 F2 mice or in reciprocal backcrosses of F1 to CBA totalling 460 animals. This apparently 'un-Mendelian' behaviour is brought about by the combined action of the following factors. While C57 Black has fairly large third molars, those of CBA are small and very variable in size, and the (potentially) smallest members of the series tend to be absent altogether. The difference in third molar size between CBA and C57 Black is multifactorial, large size being 'dominant' over small size in F1. In mice generally, the size of the third molars varies with maternal conditions which may change from pregnancy to pregnancy; while this has no further consequences where third molars are large, it leads to strikingly non-random occurrence of animals with absent molars in certain CBA litters. Third molar size also depends on the 'vigour' of the mother, young born of inbred mothers (F1) having smaller teeth than young born of hybrid mothers (F2). This leads to a delayed appearance of 'hybrid vigour' as to third molar size in F2 rather than in F1. In combination, these variables give rise to size distributions in F1, F2 and the backcross CBA $ \male $ × F1 $ \female$ in which the smaller tooth sizes of the CBA distribution do not occur at all. In the reciprocal backcross (F1 $ \male $ × CBA $ \female )$, the character 'absent molars' might be recovered if a very large sample were examined.
Proceedings of the Royal Society of London. Series B, Biological Sciences © 1951 Royal Society