Have people ever said to you, "It's in your genes?" They were probably talking about a physical characteristic, personality trait, or talent that you share with other members of your family. We know that genes play an important role in shaping how we look and act and even whether we get sick. Now scientists are trying to use that knowledge in exciting new ways, such as preventing and treating health problems.

What Is a Gene?

To understand how genes (pronounced: jeens) work, let's review some biology basics. Most living organisms are made up of cells that contain a substance called deoxyribonucleic (pronounced: dee-ahk-see-rye-bow-noo-klee-ik) acid (DNA). DNA is wrapped together to form structures called chromosomes (pronounced: krow-muh-soams).

Most cells in the human body have 23 pairs of chromosomes, making a total of 46. Individual sperm and egg cells, however, have just 23 unpaired chromosomes. You received half of your chromosomes from your mother's egg and the other half from your father's sperm cell. A male child receives an X chromosome from his mother and a Y chromosome from his father; females get an X chromosome from each parent.

So where do genes come in? Genes are sections or segments of DNA that are carried on the chromosomes and determine specific human characteristics, such as height or hair color. Because each parent gives you one chromosome in each pair, you have two of every gene (except for some of the genes on the X and Y chromosomes in boys because boys have only one of each).

Some characteristics come from a single gene, whereas others come from gene combinations. Because every person has about 25,000 per cell, there is an almost endless number of possible combinations!

Genes and Heredity

Heredity is the passing of genes from one generation to the next. You inherit your parents' genes. Heredity helps to make you the person you are today: short or tall, with black hair or blond, with green eyes or blue.

Can your genes determine whether you'll be a straight-A student or a great athlete? Heredity plays an important role, but your environment (including things like the foods you eat and the people you interact with) also influences your abilities and interests.

How Do Genes Work?

DNA contains four chemicals (adenine, thymine, cytosine, and guanine — called A, T, C, and G for short) that are strung in patterns on extremely thin, coiled strands in the cell. How thin? Cells are tiny — invisible to the naked eye — and each cell in your body contains about 6 feet of DNA thread, for a total of about 3 billion miles (if all your DNA threads were stretched out straight) of DNA inside you! The DNA patterns are the codes for manufacturing proteins, chemicals that enable the body to work and grow.

Genes hold the instructions for making protein products (like the enzymes to digest food or the pigment that gives your eyes their color). As your cells duplicate, they pass this genetic information to the new cells. Genes can be dominant or recessive. Dominant genes show their effect even if there is only one copy of that gene in the pair. For a person to have a recessive disease or characteristic, the person must have the gene on both chromosomes of the pair.

What Are Genetic Disorders?

Cells can sometimes contain changes or variants in the information in their genes. This is called gene mutation, and it often occurs when cells are aging or have been exposed to certain chemicals or radiation. Fortunately, cells usually recognize these mutations and repair them by themselves. Other times, however, they can cause illnesses, such as some types of cancer. And if the gene mutation exists in egg or sperm cells, children can inherit the mutated gene from their parents.

Researchers have identified more than 4,000 diseases that are caused by genetic variants. But having a genetic mutation that may cause disease doesn't always mean that a person will actually get that disease. Because you inherit a gene from each parent, having one disease gene usually does not cause any problems because the normal gene can allow your body to make the normal protein it needs.

On average, people probably carry from 5 to 10 variant or disease genes in their cells. Problems arise when the disease gene is dominant or when the same recessive disease gene is present on both chromosomes in a pair. Problems can also occur when several variant genes interact with each other — or with the environment — to increase susceptibility to diseases.

If a person carries the dominant gene for a disease, he or she will usually have the disease and each of the person's children will have a 1 in 2 (50%) chance of inheriting the gene and getting the disease. Diseases caused by a dominant gene include achondroplasia (pronounced: ay-kon-druh-play-zhuh, a form of dwarfism), Marfan syndrome (a connective tissue disorder), and Huntington disease (a degenerative disease of the nervous system).

People who have one recessive gene for a disease are called carriers, and they don't usually have the disease because they have a normal gene of that pair that can do the job. When two carriers have a child together, however, the child has a 1 in 4 (25%) chance of getting the disease gene from both parents, which results in the child having the disease. Cystic fibrosis (a lung disease), sickle cell anemia (a blood disease), and Tay-Sachs disease (which causes nervous system problems) are caused by recessive disease genes from both parents coming together in a child.

Some recessive genetic variants are carried only on the X chromosome, which means that usually only guys can develop the disease because they have only one X chromosome. Girls have two X chromosomes, so they would need to inherit two copies of the recessive gene to get the disease. X-linked disorders include the bleeding disorder hemophilia (pronounced: hee-muh-fih-lee-uh) and color blindness.

Sometimes when an egg and sperm unite, the new cell gets too many or too few chromosomes. Most children born with Down syndrome, which is associated with mental retardation, have an extra chromosome number 21.

In some cases, people who are concerned that they might carry certain variant genes can have genetic testing so they can learn their children's risk of having a disease. Pregnant women can also have tests done to see if the fetus they are carrying might have certain genetic illnesses. Genetic testing usually involves taking a sample of someone's blood, skin, or amniotic fluid, and checking it for signs of genetic diseases or disorders.

Changing Genes

Sometimes scientists alter genes on purpose. For many years, researchers have altered the genes in microbes and plants to produce offspring with special characteristics, such as an increased resistance to disease or pests, or the ability to grow in difficult environments. We call this genetic engineering.

Gene therapy is a promising new field of medical research. In gene therapy, researchers try to supply copies of healthy genes to cells with variant or missing genes so that the "good" genes will take over. Viruses are often used to carry the healthy genes into the targeted cells because many viruses can insert their own DNA into targeted cells.

But there are problems with gene therapy. Scientists haven't yet identified every gene in the human body or what each one does. Huge scientific efforts like The Human Genome (pronounced: jee-nome) Project and related projects have recently completed a map of the entire human genome (all of the genetic material on a living thing's chromosomes), but it will take many more years to find out what each gene does and how they interact with one another. For most diseases, scientists don't know if and how genes play a role. Plus, there are major difficulties inserting the normal genes into the proper cells without causing problems for the rest of the body.

There are also concerns that people might try changing genes for ethically troubling reasons, such as to make smarter or more athletic children. No one knows what the long-term effects of that kind of change would be.

Still, for many people who have genetic diseases, gene therapy holds the hope that they — or their children — will be able to live better, healthier lives.

Reviewed by: Louis E. Bartoshesky, MD, MPH
Date reviewed: April 2009