Genes determine such features as the shape of a leaf or the sex, height, and hair color of a child. Every cell has tens of thousands of genes, which occur on threadlike structures called chromosomes.
Most genes occupy a specific place on a certain chromosome.
Genes cannot be seen with an ordinary light microscope. Scientists investigate the locations and properties of genes with advanced laboratory methods. Genes produce their effects by influencing chemical and physical processes that control how living things grow, develop, and function. Each gene is a particular segment of a DNA molecule in a chromosome. Daniel L. Hartl, Ph.D., Professor of Organismic and Evolutionary Biology, Harvard University.