A gene is one of the units that determine the characteristics an organism inherits from its ancestors. Such inherited characteristics include height and the color of the skin, eyes, and hair. Some traits, such as a person's ability to curl his tongue, are determined by a single gene, and others, such as skin color, are determined by a combination of genes.
Every cell has thousands of genes. They are located on the chromosomes, which are small thread like structures in the nucleus, and each gene occupies a specific place on a chromosome. The number of genes and the way in which they are arranged on the chromosomes are always the same in every member of a species.
Since chromosomes usually occur in pairs, genes also are paired. The genes that have a similar location on each chromosome in a pair are called alleles. Alleles influence the same hereditary trait, but they may have a different effect on it. For example, the gene that produces short plants is an allele of the gene that produces tall plants. Each time a cell divides, every one of its genes makes a copy of itself. Occasionally a gene is changed in some way so that it has a different effect on a trait. Such a change in a gene is called a mutation. The original gene and the altered gene are alleles.
Genes are too tiny to be seen, but biochemical studies have shown that they consist of nucleic acids combined with protein. It is believed that they exert their influence through the enzymes in cells. Each gene probably controls the synthesis of a specific protein. The protein, in turn, acts as an enzyme that makes possible a particular reaction in the cell. The reaction may be concerned with protein synthesis, growth, pigment formation, or any other activity in the cell.