Genetic imbalances—such as those associated with Down syndrome in humans—are generally harmful. The number of copies of a given gene, the gene dose, can be very important. In the roundworm C. elegans, sex (male or hermaphrodite) is determined by the number of X chromosomes. Dr. Barbara Meyer explains how hermaphrodites control gene expression on their pair of X chromosomes to avoid having double the gene dose of males. By studying these sophisticated genetic systems in worms, scientists can learn more about related molecular pathways in mammals and humans. From the Series:HHMI’s Understanding Biomedical Research Series