What are genes?

Genes are pieces of DNA (deoxyribonucleic acid) inside each cell that tell the cell what to do and when to grow and divide. Each gene is made up of a specific DNA sequence that contains the code (the instructions) to make a certain protein, each of which has a specific job or function in the body. Each human cell has about 25,000 genes.

Most genes are contained in chromosomes. A chromosome is a long strand of DNA wrapped around a special protein called histone. Most chromosomes contain many different genes. Most human cells contain 23 pairs of chromosomes – one pair of sex chromosomes (either XX in females or XY in males) plus 22 pairs of non-sex chromosomes called autosomes. Sperm and egg cells only contain half as many chromosomes (23). Chromosomes are passed from parents to their children through sperm and egg cells. One chromosome of each pair is inherited from the mother, and the other comes from the father. This is why children look like their parents, and why they may have a tendency to develop certain diseases that run in their families.

A cell uses its genes selectively; that is, it can turn on (or activate) the genes it needs at the right moment and turn off other genes that it doesn't need. All the cells in the body (except egg and sperm) contain the same genes. Turning on some genes and turning off others is how a cell becomes specialized. That is how a cell becomes a muscle cell and not a bone cell, for example. Some genes stay active all the time to make proteins needed for basic cell functions. Others shut down when their job is finished and start again later if needed.

Dominant vs. recessive genes

We have 2 versions (copies) of most genes – one from each parent. For some versions of a gene, only one copy is needed to see a certain quality or disease (in genetics this is called a trait). These genes are called dominant. If both copies have to be the same to see that trait, it is called recessive. For example, the gene for brown eyes is dominant while the gene for blue eyes is recessive, so if you get one copy of the brown eye gene from one parent and a copy of the blue eye gene from the other, you will have brown eyes. You will only get blue eyes if you get 2 copies of the blue eye gene (one from each parent). This classification applies to gene mutations as well. If you only need to inherit one copy of a gene mutation to get a disease or syndrome, it is called dominant. If you need 2, it is called recessive. (Gene mutations are discussed in the next section.)

X-linked genes

Things are a little different in terms of genes on the X chromosome. Normally, we each have 2 sex chromosomes. Women have two X chromosomes, while males have one X chromosome and one Y chromosome. Since the Y chromosome contains different genes than the X chromosome, males have only one copy of the genes on the X chromosome. Some diseases/conditions are caused by genes on the X chromosome. For some of these, like color blindness, a female has to have 2 copies of the gene (one on each X chromosome) to get the condition. For a male though, he only has to have the gene on his one X chromosome. Diseases and conditions like this are called X-linked. X-linked conditions are more common in males.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: June 25, 2014 Last Revised: June 25, 2014

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