Gene The smallest subunit of a chromosome that contains a genetic message. Gene Mutation A change in the DNA sequence within a gene. Genetic Toxicity Toxic effects that result from damage to DNA and altered genetic expression. Genotoxic see genetic toxicity. Germ Cell Reproductive cells which give rise to sperm or ova. Glomerulus The highly vascular structure in the kidney where much of the fluid portion of the blood (serum) is filtered and passes into the kidney tubules, carrying with it toxins and many other materials present in the serum.