The sex and sexual characteristics constitute key aspects of an organism’s life and are determined by a biological process known as sex determination. These ever-evolving mechanisms are broadly classified based on the type of “switch” that triggers them. Genetic sex determination is dependent on sex chromosomes, such as the X and Y chromosomes in human beings, whereas environmental sex determination depends on factors like temperature and the local ratio between males and females. Although most sex determination mechanisms are genetic or environmental, a third type of sex determination, which depends on completely random factors, also exists. This, however, has not been explored completely. The sex determination mechanism of Caenorhabditis elegans, a species of nematode, or our common garden-variety roundworm, is one of the best understood aspects of its biology. In its case, embryos with two X chromosomes, or the XX embryos, develop into hermaphrodites, while the XO embryos, which have one sex chromosome—the X chromosome—develop into males. Several species of nematodes have a sex determination mechanism similar to that of C. elegans. Interestingly, however, some nematode species also rely on the XX/XY system for sex determination, with both X and Y types of sex chromosomes, as well as on environmental factors. Unfortunately, the mechanisms that cause this variance in sex-determination between nematode species have remained a mystery thus far.