Genes are the basic units of inheritance.
They are made of a chemical called DNA, or deoxyribonucleic acid. Many traits are partly influenced by the genes and partly by the environment. For example, people sometimes change their hair color by using dyes or highlighting, or change their eye color by wearing tinted contact lenses. So sometimes it is hard to tell whether a given characteristic is determined by genes, environment, or both . Human beings have approximately 100,000 pairs of genes.