Kyphoscoliosis in a Long-Term Selection Experiment with Chickens

Abstract

and scoliosis was added to the selection criteria for the crossbred lines and for the Leghorn line, with relaxation of the shank-length criterion. In the White crossbred line, stocks with scoliotic incidences of over 50% and of less than 5% were developed by divergent selection pressure. Additional pressure for kyphoscoliosis in the Black crossbred line nearly caused extinction of the stock, with no increase in scoliotic incidence. The incidence of scoliosis decreased in the Size Leghorn line. Reciprocal outcrosses between scoliotic chickens of White and Size lines produced F1 generations with no scoliosis, but the F2 to F5 generations developed variably low levels of the defect. One outcross of a kyphotic Size $ with scoliotic White Y 9 produced a low F1 scoliotic incidence. Crosses between normal, kyphotic, and scoliotic or kyphoscoliotic phenotypes produced intermediate levels of kyphosis and scoliosis, the former being at higher incidences. Selection within phenotypes was relatively more effective in normal than in scoliotic matings. No evidence was obtained for operation of a major gene controlling scoliosis. Data from matings within phenotypes and a high association of kyphosis with scoliosis indicated a sharing of some genes in development of the characters. A complex sytem of interacting modifying genes, some sex-linked, operating in a highly selective genetic background obtainable by inbreeding appears to be involved in the inheritance of kyphoscoliosis. The utility of the White normal and scoliotic stocks for further studies is discussed.