Genes (say: jeenz) play an important role in determining physical traits — how we look —and lots of other stuff about us. They carry information that makes you who you are and what you look like: curly or straight hair, long or short legs, even how you might smile or laugh. Many of these things are passed from one generation to the next in a family by genes.

What Is a Gene?

Genes carry the information that determines your traits (say: trates), which are features or characteristics that are passed on to you — or inherited — from your parents. Each cell in the human body contains about 25,000 to 35,000 genes. 

For example, if both of your parents have green eyes, you might inherit the trait for green eyes from them. Or if your mom has freckles, you might have freckles too because you inherited the trait for freckles. Genes aren't just found in humans — all animals and plants have genes, too.

Where are these important genes? Well, they are so small you can't see them. Genes are found on tiny spaghetti-like structures called chromosomes (say: KRO-moh-somes). And chromosomes are found inside cells. Your body is made of billions of cells. Cells are the very small units that make up all living things. A cell is so tiny that you can only see it using a strong microscope.

Chromosomes come in matching sets of two (or pairs) and there are hundreds — sometimes thousands — of genes in just 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.

Most cells have one nucleus (say: NOO-clee-us). The nucleus is a small egg-shaped structure inside the cell which acts like the brain of the cell. It tells every part of the cell what to do. But, 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.

Under the microscope, we can see that chromosomes come in different lengths and striping patterns. When they are lined up by size and similar striping pattern, the first twenty two of the pairs these are called autosomes; the final pair of chromosomes are called sex chromosomes, X and Y. The sex chromosomes determine whether you're a boy or a girl: females have two X chromosomes while 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. The DNA in a gene spells out specific instructions—much like in a cookbook recipe — 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, are all made up of proteins. 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 more than 300,000 proteins!

Like chromosomes, genes also come in pairs. Each of your 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 Emma's mother has one gene for brown hair and one for red hair, and she passed the red hair gene on to Emma. If her father has two genes for red hair, that could explain her red hair. Emma ended up with two genes for red hair, one from each of her parents.

You also can see genes at work if you think about all the many different breeds of dogs. 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. They want to know which proteins each gene makes and what those proteins do. They also want to know what illnesses are caused by genes that don't work right. Genes that have been changed are called mutations. Researchers think that mutations may be partly to blame for lung problems, cancer, and many other illnesses. Other illnesses and health problems happen when there are missing genes or extra parts of genes or chromosomes.

Some of these gene problems can be inherited from a parent. For example, take the gene that helps the body make hemoglobin (say: HEE-muh-glow-bin). Hemoglobin is an important protein needed for red blood cells to carry oxygen throughout the body. If parents pass on altered hemoglobin genes to their child, the child might 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. 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 a changed 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're sick — that gets stuck in the lungs. People with CF need treatment throughout their lives to keep their lungs as healthy as possible.

What Is Gene Therapy?

Gene therapy is a new kind of medicine — so new that scientists are still doing experiments to see if it works. It uses the technology of genetic engineering to treat a disease caused by a gene that has changed in some way. One method being tested 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.