Selection for an Invariant Character , Vibrissa Number , in the House Mouse

k y canalised character the genotype may be considered as having two Fysiects: firstly, the genes controlling the potential variability of the character and secondly, the genes which act during development to canalise or limit that variability. That a normally invariant character may have underlying genetic variability has been demonstrated for a number of characters ( WADDINGTON 1953; RENDEL 1959; DUN and FRASER 1959) but the actual genetic mechanism by which underlying variability is channelled into phenotypic invariance has received less attention. FRASER (1960) has shown by means of a computer model how an epistatic genotype could work to produce a curvilinear genotype-phenotype relationship, i.e., a zone of canalisation in which change in the genotype dose does not result in phenotypic change. RENDEL and SHELDON (1960) have actually produced a new canalisation zone by selecting, in a scute stock of Drosophila which normally shows canalisation at four scutellar bristles, for low variance in number of scutellar bristles around a mean of two bristles. The shifting of a character away from a canalisation zone can be achieved either by selection for a basic genotype which is so extreme that the canalisation system cannot cope with it, or by selecting for altered canalisation. Such an alteration may be a change in the intensity of canalisation or a change in the level of expression at which canalisation occurs. If selection is carried out simply for high or low expression of the character and if both genetic systems are available for selection, then it is to be expected that both systems will respond to a greater or lesser degree (WADDINGTON 1955). The secondary vibrissae of mice are canalised at a total of 19 (DUN and FRASER 1959). However, the Tabby gene reduces the number of these vibrissae and at the same time increases their variance. Selection on this exposed variation has produced a marked response not only in Tabby mice but also in non-Tabby sibs (DUN and FRASER 1959; FRASER and KINDRED 1960). A probit analysis of the data from this and other selection experiments was undertaken to compare the intensity of canalisation of different lines and to detect any change in the pattern of canalisation which may have accompanied the change in mean vibrissae score.