What is Genetics? : What are genes?

What are genes?

To organize the huge amount of information, our DNA is made up of segments called genes, all with specific jobs.

We all have over 25,000 genes! Everyone has the same genes, but it is the variation (the changes) in those genes that make us different.You could think of our genes as the separate recipes inside our cookbooks. Everyone has the same recipes, but some of the ingredients or the amount of ingredients may be different from person to person. Each gene (or recipe) has instructions for a specific section or function of our body. For example, we have recipes that tell our bodies:

  • what color our eyes are
  • how tall we are
  • how our bodies digest sugar

How do genes cause DISEASE?

Our genes are not meant to cause disease. The vast majority of our genes work to help our bodies grow and function as a healthy person all the way from pregnancy to adulthood. It is when there is an abnormal change in a gene that causes a person to have a disease or causes a person to be at risk for getting a disease.

Sometimes, a gene change can cause a disease directly. For example, Huntington disease is caused by a specific change in the HD gene. This type of change is called a mutation because it causes the gene to function incorrectly, thereby causing a disease. There are no other causes for Huntington disease; if a person has the mutation, they will develop the disease. Other changes in our DNA only cause a person to be at risk for developing a disease. We call these variations, or polymorphisms . For example, people with certain polymorphisms in the APOE gene are at increased risk for developing Alzheimer disease. People with this specific change may never develop Alzheimer disease, and people who don’t have that change may get Alzheimer disease without it. This shows us that although our genes can tell us if we are at risk for many diseases, they don’t always predict if we will definitely develop the disease. Our environment and lifestyle along with our genes is very important in predicting our risk for disease. To learn more about how our environment and our genes interact to cause disease, visit the Complex Disease page.

How are GENES passed on through the family?

You get half of your genes from your mother and half from your father. Since your genes determine a lot of what you look like, you look like both of your parents, but you are a completely unique mix. You also pass on half of your genes to your children. But there is no way to choose which genes you can pass on. Genes can be passed down through a family in different inheritance patterns. Examples are autosomal dominant, autosomal recessive, or X-linked. Many genetic syndromes that are caused by gene mutations follow one of these inheritance patterns.

Autosomal recessive inheritance pattern

Cystic fibrosis (CF) is a genetic disease of the lungs and digestive system caused by mutations in the CFTR gene. It has an autosomal recessive inheritance pattern. This means that both parents must be carriers of one CFTR mutation to have a child with cystic fibrosis. Parents who are carriers of cystic fibrosis have a 25% chance of having a child who has the disease.

Autosomal dominant inheritance pattern

There are many diseases that follow this inheritance pattern. To have an autosomal dominant condition, a person has a mutation in only one copy of the gene associated with the disease. For example, Marfan syndrome is an autosomal dominant genetic condition that causes abnormalities of the joints, heart, and eyes. People with the condition are usually very tall and thin. People with Marfan syndrome can live a normal, healthy life, but there is a higher risk for a dangerous heart complication, so they must see a doctor regularly. A person with Marfan syndrome has a mutation in one copy of their FBN1 gene. This mutation causes the FBN1 gene to not function normally, causing the features of the condition. When a person with Marfan syndrome has children, they can pass on either copy of their FBN1 gene: the one with the mutation or the one without the mutation. If their child inherits the copy with the mutation, they will have Marfan syndrome as well. If they inherit the copy without the mutation, they will not have Marfan syndrome. Therefore, for each child, there is a 50% chance they will have Marfan syndrome.

X-linked inheritance pattern

A final type of inheritance pattern is called X-linked. It is called X-linked because the gene with the mutation that causes the disease is located on the X chromosome. Girls have two copies of the X chromosome, while boys have only one. They have a Y instead of a second X. Therefore, a girl with a non-working gene (due to a mutation) on one of her X chromosomes may not be affected, because they have a second X chromosome to make up for the non-working gene. But a boy with the same non-working gene on his one X chromosome will be affected by the condition. This is because he doesn’t have a second copy of the gene to make up for the non-working one. An example of an X-linked condition is a muscle disease called Duchenne muscular dystrophy (DMD). Typically, boys only get DMD. A boy with DMD has a mutation in the DMD gene on his X chromosome. A girl who has the same mutation on her X chromosome will not have the disease, because she has a second copy of the working DMD gene on her other X chromosome. Girls in this case will be carriers. A woman who is a carrier is at risk of having children with DMD. There is a 50% chance for each of her children to inherit the X chromosome with the non-working DMD gene. If her child is a boy and he inherits the non-working gene, he will have DMD.

To learn more about how diseases can pass on through a family, visit the My Family Health History page.

Read on: What is genetic testing?