This program illustrates the experimental techniques involved in the genetic analysis of an ascospore color mutant in Sordaria. Two self-sterile but interbreeding strains are used. Mutations are induced by ultraviolet irradiation which result in the inability to form the black melanin pigment normally present in the ascospore. The colors of the ascospores can be seen through the transparent wall of the ascus before discharge; the linear arrangement of the ascospores makes it possible to identify the products of the first and second divisions of meiosis and the subsequent mitosis. By mating normal with mutant strains, various combinations of yellow and black appear—but always four yellow and four black. In working out the patterns for locating the single mutant gene, students will be utilizing some of the fundamental laws of all genetics. (15 minutes)