Chromosomes are threadlike nuclear structures consisting of DNA and proteins that serve as the repositories for genetic information. The chromosomes depicted here were isolated from a fruit fly’s salivary gland, stained with dye, and visualized under a microscope. Akin to miniature bar codes, chromosomes absorb different dyes to produce characteristic banding patterns, which allows for their routine identification. (credit: modification of work by “LPLT”/Wikimedia Commons; scale-bar data from Matt Russell)
chromosomes isolated from a fruit fly’s salivary gland

According to the United Nations Office on Drugs and Crime, approximately 95% of those who commit homicide are men. While behavior is shaped by the environment one grows up and lives in, genetics also play a role. For example, scientists have discovered genes that appear to increase one’s tendency to exhibit aggressive behavior. One of the genes, called MAOA, is located on the X chromosome. In one recent study involving a group of male prisoners in Finland, scientists found that the prisoners who inherited a variant of this gene were between 5% and 10% more likely to have committed a violent crime. Men only have one copy of the gene, since men only have one X chromosome. Women, however, have two copies of the X chromosome and therefore two copies of the gene. Therefore, women who inherit the variant allele will most likely also have a normal allele to counteract its effects. It is important to note that many men inherit the variant copy of MAOA and only some commit violent crimes. The environment seems to play a much more critical role.

Long before chromosomes were visualized under a microscope, the father of modern genetics, Gregor Mendel, began studying heredity in 1843. With the improvement of microscopic techniques during the late 1800s, cell biologists could stain and visualize subcellular structures with dyes and observe their actions during cell division and meiosis. With each mitotic division, chromosomes replicated, condensed from an amorphous (no constant shape) nuclear mass into distinct X-shaped bodies (pairs of identical sister chromatids), and migrated to separate cellular poles.

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