Penetrance of a rare genetic defect

Abstract

Contrary to the conclusion of earlier workers, penetrance of a very rare genetic defect, congenital tremor in White Leghorn chicks, was found to be complete. By appropriate correction of the original data for small family size it is shown that the defect is determined not by one but two pairs of autosomal recessive genes. In the terminology of human genetics, ascertainment of affected individuals corresponded to conditions of ‘truncate’ selection in the case of maternal sibships and more nearly to ‘single’ selection in the case of paternal sibships.