The doorbell rings. Busy in the kitchen fixing dinner, Nancy's dad calls out, "Answer the door, Nancy! My hands are full!" Nancy opens the front door, and suddenly a bunch of people she hasn't seen in 3 years pour into the house.

Aunt Rita hands Nancy a wrapped package and says, "Well, look at you! How you've grown. And you've got such beautiful red, curly hair! It runs in the family, you know. You look just like my grandmother!" Uncle Michael adds, "And she's going to be tall, like her father. Only 9 years old, and she looks like a basketball player already!"

Nancy makes a dash to the kitchen, wondering, "Huh? Aunt Rita's grandmother? Runs in the family? Basketball? What are they talking about?"

Genes (say: jeenz), that's what they're talking about. Genes are the things that play an important role in determining physical traits — how we look — and lots of other stuff about us. They carry information that helps make you who you are: curly or straight hair, long or short legs, even how you might smile or laugh, are all passed through generations of your family in genes.

Keep reading to learn more about genes and how they work.

What Is a Gene?

Each cell in the human body contains about 25,000 to 35,000 genes, which carry information that go toward determining your traits (say: trates). Traits are characteristics you inherit from your parents; this means your parents pass some of their characteristics on to you through genes. For example, if both of your parents have green eyes, you might inherit the trait of green eyes from them. Or if your mom has freckles, you might inherit that trait and wind up with a freckled face. And genes aren't just in humans — all animals and plants have genes, too.

Genes hang out all lined up on thread-like things called chromosomes (say: kro-moh-somes). Chromosomes come in pairs, and there are hundreds, sometimes thousands, of genes in one chromosome. The chromosomes and genes are made of DNA, which is short for deoxyribonucleic (say: dee-ox-see-ri-bo-nyoo-clay-ik) acid.

Chromosomes are found inside cells, the very small units that make up all living things. A cell is so tiny that you can only see it through the lens of a strong microscope, and there are billions of cells in your body. Most cells have one nucleus (say: noo-clee-us). The nucleus, which is sort of egg-shaped, is like the brain of the cell. It tells every part of the cell what to do. How does the nucleus know so much? It contains our chromosomes and genes. As tiny as it is, the nucleus has more information in it than the biggest dictionary you've ever seen.

In humans, a cell nucleus contains 46 individual chromosomes or 23 pairs of chromosomes (chromosomes come in pairs, remember? 23 x 2 = 46). Half of these chromosomes come from one parent and half come from the other parent. Twenty two of the pairs appear identical (these are called autosomes); the other two chromosomes are called sex chromosomes — X and Y. Females have 2 X chromosomes; males have one X and one Y. But not every living thing has 46 chromosomes inside of its cells. For instance, a fruit fly cell only has four chromosomes!

How Do Genes Work?

Each gene has a special job to do. It carries blueprints — the instructions — for making proteins (say: pro-teens) in the cell. Proteins are the building blocks for everything in your body. Bones and teeth, hair and earlobes, muscles and blood, all are made up of proteins (as well as other stuff). Those proteins help our bodies grow, work properly, and stay healthy. Scientists today estimate that each gene in the body may make as many as 10 different proteins. That's over 300,000 proteins!

Like chromosomes, genes come in pairs. Each of your biological parents has two copies of each of their genes, and each parent passes along just one copy to make up the genes you have. Genes that are passed on to you determine many of your traits, such as your hair color and skin color.

Maybe Nancy's mother has one gene for brown hair and one for red hair, and she passed the red hair gene on to Nancy. If her father has two genes for red hair, that could explain her red hair. Nancy ended up with two genes for red hair, one from each of her parents.

You can see genes at work if you think about all the breeds of dogs there are. They all have the genes that make them dogs instead of cats, fish, or people. But those same genes that make a dog a dog also make different dog traits. So some breeds are small and others are big. Some have long fur and others have short fur. Dalmatians have genes for white fur and black spots, and toy poodles have genes that make them small with curly fur. You get the idea!

When There Are Problems With Genes

Scientists are very busy studying genes. What do the proteins that each gene makes actually do in the body? What illnesses are caused by genes that don't work right? Researchers think genes that have changed in some way, also known as altered (or mutated) genes, may be partly to blame for lung problems, cancer, and many other illnesses.

Take the gene that helps the body make hemoglobin (say: hee-muh-glow-bin), for example. Hemoglobin is an important protein that is needed for red blood cells to carry oxygen throughout the body. If parents pass on altered hemoglobin genes to their child, the child may only be able to make a type of hemoglobin that doesn't work properly. This can cause a condition known as anemia (say: uh-nee-mee-uh), a condition in which a person has fewer healthy red blood cells.

Anemias that are inherited can sometimes be serious enough to require long-term medical care. Sickle cell anemia is one kind of anemia that is passed on through genes from parents to children.

Cystic fibrosis (say: sis-tick fi-bro-sus), or CF, is another illness that some kids inherit. Parents with the CF gene can pass it on to their kids. People who have CF often have trouble breathing because their bodies make a lot of mucus (say: myoo-kus) — the slimy stuff that comes out of your nose when you blow — that gets stuck in the lungs. They will need treatment throughout their lives to keep their lungs as healthy as possible.

What Is Gene Therapy?

Gene therapy uses the technology of genetic engineering to cure or treat a disease caused by a gene that has changed in some way. This is a new kind of medicine, and scientists are still doing experiments to see if it works. One method they are trying is replacing sick genes with healthy ones. Gene therapy trials — where the research is tested on people — and other research may lead to new ways to treat or even prevent many diseases.

Reviewed by: Louis E. Bartoshesky, MD, MPH
Date reviewed: June 2010