Chapter 12

Microphthalmia and Other Considerations

II. Other Determinants Associated with White Spotting

A. Fleck ( Fk)

Fleck is an autosomal dominant which is lethal when homozygous. The mutation, which has not as yet been mapped, was first observed by Sheridan ( 1968) and what follows is based on his account. Fleck arose in the CBA strain. It is characterized by a white belly spot, white tail tip and often white back paws. Although animals expressing this completely penetrant character are both viable and fertile, when mutant males are mated to mutant females the rate of intrauterine death at the time of implantation is about 25% greater than when mutant females are mated to normal males, an observation which suggests that the Fk/Fk genotype is lethal.

In spite of the fact that there is no indication the Fk when heterozygous affects the well being or life expectancy of its carriers, it does influence their breeding performance. Thus Fk/+ males had a significantly lower mating prowess than their normal brothers when mated to normal CBA females. The size of the first and second (but not the third) litters of mutant females mated to normal CBA males also was smaller than their normal sisters. Moreover, the Fk/+ mothers were more likely to lose their litters completely. Whether this is due to cannibalism, or to the inability of the parent to care for the litter, has not yet been determined but it is consistent with the observation that smaller litters are more apt to be completely lost ( Luning, cited by Sheridan). The reduction in litter size from fleck females could be due to any one of several different factors or some combination of them: for example, a smaller number of eggs ovulated, a reduced rate of implantation, or an increased rate of intrauterine death. ²⁷

Despite the higher prenatal mortality in the first two litters of Fk/+ female x +/+ male matings, there is no evidence that this mortality is directed against the heterozygous embryos. Indeed, the fact that the data from weaned litters showed a good agreement with the expected 1:1 ratio argues against such selection.

Finally, although Fk in some respects mimics the effect of splotch ( Sp) no evidence was found in either dissection studies or in progeny tests that these mutants are allelic. Fk has been shown also not to be allelic with viable dominant spotting ( W^v) or steel ( Sl^d).

B. Belly Spot and Tail ( B^st)

This dominant mutation (linkage unknown) is of recent origin ( Southard and Eicher, 1977) and its gene symbol is therefore provisional. It occurred in strain C57BL/KsJ and causes a white belly spot, a short, kinky tail, and a polydactylous condition affecting one to three feet. Both sexes are fertile and viable.