The first Meckel oration: on the causes of morphological differences in a population of guinea pigs.

The first morphological abnormality considered here was of a very superficial sort: deviations from the smooth coat of wild cavies and most guinea pigs. The deviations of "rough-furred" individuals range from a single irregularity or a single pair of rosettes to the "full rough" pattern. Roughness may be restricted to the hind toes, forehead, or hair around the eyes or belly, or (very rarely) to a small area on one side of the back. There is no indication of gene control in the last, and no analysis of the only strain (in Professor Castle's laboratory) that I have seen, of roughness restricted to the belly. A dominant gene, R, in combination with a semidominant modifier, M, account for the fancier roughs (R-MM toes only, R-mm full rough). A gene, st, that is completely dominant in the absence of R, semidominant in its presence, was descended from 3 animals from outside the colony. It accounts for a large forehead rosette, usually with a small white spot in front of its center. A statistically semidominant gene, Re, with extremely irregular, often asymmetric penetrance tends to cause rosettes about the eyes. This arose by mutation late in the history of the Whitman colony. It is strengthened by presence of R MM. Considerable variation occurs in the numbers of dorsal rosettes of R Mm and R mm. These have not been analyzed satisfactorily because the genetic differences are confounded by nongenetic ones. It is not practicable to summarize briefly all of the complex interaction effects of these factors, but it may be noted that the most surprising one had to do with genes St and R. The forehead rosette due to rr St closely resembles that of R mm (except for the absence of the pleiotropic white spot). The combination, R mm St St, shows none of the expected enhancement but instead shows nearly complete cancellation of the rosette. Moreover, the anterior dorsal rosettes of R mm st st are much reduced and seemingly shoved backward and laterally to give a large smooth shield back of the ears. St St also reduces the usual dorsal pair of rosettes of R Mm. With St st, all of these antagonistic effects are weaker and less regular. Another morphological deviation to be considered was the fairly common restoration of an atavistic little toe.(ABSTRACT TRUNCATED AT 400 WORDS)