Studies on Dominance and Pleiotropy Using Segregating Inbred Lines of Fowl.

R. A. FISHER (1930) developed a phylogenetic and statistical theory of dominance of wild type over mutants involving a selection of modifiers that shift the phenotype of the heterozygous mutant toward the wild type. The wild type has a selective advantage which also is conferred upon heterozygotes. FISHER (1 934,1935,1938) noted that many common mutations in the domestic fowl were dominant in contrast to those of Drosophila. The fowl, therefore, provided material to test a crucial point in the dominance theory. If “domestic” dominants had evolved through artificial selection during the process of domestication, this could be demonstrated by showing that, in wild stocks not previously subjected to artificial selection, dominance is incomplete; i.e., the heterozygote is intermediate between two homozygous types. FISHER’S experiment consisted of introducing several supposedly dominant genes into a stock of wild jungle fowl. This was done by backcrossing the heterozygote five successive generations to the jungle fowl stock followed by interbreeding the heterozygotes. Thus, all three genotypes were obtained in a relatively pure jungle-fowl genetic background. Dominance was found to be incomplete in the case of three mutations influencing pigmentation. These were, Barring ( B ) , pile (dominant white) ( I ) , and black internal pigment. In fact, barring appeared to be more nearly recessive than dominant. Later, FISHER (1949) in his book, Theory of Inbreeding, suggested that segregating inbred lines might be useful in studies of dominance and linkage and that “What is needed in practice is the gene and at least one of its allelomorphs-segregating together in a uniform genetic background, and this is supplied by a segregating inbred line.-The time consumed in preparing the genetic material for making an adequate comparison is often considerable and it is a great advantage to possess stocks of prepared material available for future use.” DUNN’S (1942) investigation of two reduced tail-length mutations ( T ) and (Sd) in the mouse is a notable example of segregating inbred lines and dominance