National Cancer Institute 1. Genes are the biological units of heredity. Genes determine obvious traits, such as hair and eye color, as well as more subtle characteristics, such as the ability of the blood to carry oxygen. Complex traits, such as physical strength, may be shaped by the interaction of a number of different genes along with environmental influences. Humans have between 50,000 and 100,000 genes.
Genes carry instructions that allow the cells to produce specific proteins such as enzymes.
Only certain genes in a cell are active at any given moment. As cells mature, many genes become permanently inactive. The pattern of active and inactive genes in a cell and the resulting protein composition determine what kind of cell it is and what it can and cannot do. Flaws in genes can result in disease. Advances in understanding and manipulating genes have set the stage for scientists to alter patients' genetic material to fight or prevent disease.