The Dunham group studies the costs and benefits of changes in gene and chromosome copy number (aneuploidy) in S. cerevisiae. Gene copy number change is a common evolutionary mechanism, and is often seen in cancers. Previously, the lab carried out a series of experiments to determine the consequences of changing the copy number of every gene in the yeast genome. 4383 yeast strains, each carrying a plasmid containing a yeast gene and a unique barcode sequence, were competed in nutrient-limited continuous-culture environments. The relative success of each strain was found by performing high-throughput sequencing on the barcodes. Our project explored three assumptions of this experiment, using direct chemostat competition assays. First, we confirmed that fitness effects observed in the pooled experiment could be repeated in individual experiments. We performed competition experiments using 22 genes from the experiment that were found to either be beneficial in certain environments or potentially harmful when duplicated. We also found that the plasmid alone had a fitness effect, by competing a strain containing a plasmid without a gene insert against a wild-type strain. Finally we tested the fitness effects of Autonomously Replicating Sequences (ARSs), which are origins of DNA replication found in many of the loci we are investigating. In a competition between a strain transformed with a plasmid with two ARSs and a strain with a plasmid containing a single ARS, a fitness benefit was observed for the presence of an extra ARS on the plasmid. These data are important controls for the pooled experiments previously performed in the Dunham lab and will help to identify the precise molecular causes of fitness changes associated with aneuploidy, and, perhaps in the future, provide some new insights into cancer biology.