Genes are the biochemical units of heredity that make up chromosomes, the threadlike coils of DNA. When genes are turned on (expressed), they provide the code for creating the proteins that form our body's building blocks. Most human traits are influenced by many genes acting together. The human genome is the shared genetic profile that distinguishes humans from other species, consisting at an individual level of all the genetic material in an organism's chromosomes. Behavior geneticists study the relative power and limits of genetic (our heredity) and environmental influences on behavior.