Genetics

When we are born, our body comes with a set of instructions, half of them from our mother and half from our father. The ways those instructions combine and interact affect many things, from our eye and hair color to our susceptibility to disease. This is what genetics is all about, those instructions.

Deoxyribonucleic acid (DNA) is the chemical compound that contains the instructions needed to develop and direct the activities of nearly all living organisms. DNA molecules are made of two twisting, paired strands, often referred to as a double helix.

Each DNA strand is made of four chemical units, called nucleotide bases, which comprise the genetic "alphabet." The bases are adenine (A), thymine (T), guanine (G), and cytosine (C). Bases on opposite strands pair specifically: an A always pairs with a T; a C always pairs with a G. The order of the As, Ts, Cs and Gs determines the meaning of the information encoded in that part of the DNA molecule just as the order of letters determines the meaning of a word.

Our complete set of DNA is called our genome. Virtually every single cell in the body contains a complete copy of the approximately 3 billion DNA base pairs, or letters, that make up the human genome.

With its four-letter language, DNA contains the information needed to build the entire human body. A gene traditionally refers to the unit of DNA that carries the instructions for making a specific protein or set of proteins. Each of the estimated 20,000 to 25,000 genes in the human genome codes for an average of three proteins.

Located on 23 pairs of chromosomes packed into the nucleus of a human cell, genes direct the production of proteins with the assistance of enzymes and messenger molecules.

People get (inherit) their chromosomes, which contain their genes, from their parents. Chromosomes come in pairs and humans have 46 chromosomes, in 23 pairs. Children randomly get one of each pair of chromosomes from their mother and one of each pair from their father. The chromosomes that form the 23rd pair are called the sex chromosomes. They decide if a person is male or female. A female has two X chromosomes, and a male has one X and one Y chromosome. Each daughter gets an X from her mother and an X from her father. Each son gets an X from his mother and a Y from his father.

Genetic disorders

Genetic disorders can happen for many reasons. Genetic disorders often are described in terms of the chromosome that contains the gene. If the gene is on one of the first 22 pairs of chromosomes, called the autosomes, the genetic disorder is called an autosomal condition. If the gene is on the X chromosome, the disorder is called X-linked.

Genetic disorders also are grouped by how they run in families. Disorders can be dominant or recessive, depending on how they cause conditions and how they run in families.

• Dominant - dominant diseases can be caused by only one copy of a gene with a DNA mutation. If one parent has the disease, each child has a 50 percent chance of inheriting the mutated gene.
• Recessive - For recessive diseases, both copies of a gene must have a DNA mutation in order to get one of these diseases. If both parents have one copy of the mutated gene, each child has a 25% chance of having the disease, even though neither parent has it. In such cases, each parent is called a carrier of the disease. They can pass the disease on to their children, but do not have the disease themselves.

Some genetic diseases are caused by a DNA mutation in one of a person's genes. For example, suppose part of a gene usually has the sequence TAC. A mutation can change the sequence to TTC in some people. This change in sequence can change the way that the gene works, for example by changing the protein that is made.

Mutations can be passed down to a child from his or her parents. Or, they can happen for the first time in the sperm or egg, so that the child will have the mutation but the parents will not. Single gene disorders can be autosomal or X-linked.

For example, sickle cell disease is an autosomal single gene disorder. (Read about "Sickle Cell Disease") It is caused by a mutation in a gene found on chromosome 11. Sickle cell disease causes anemia and other complications. (Read about "Anemia") Fragile X syndrome (Read about "Fragile X Syndrome"), on the other hand, is an X-linked single gene disorder. It is caused by a change in a gene on the X chromosome. It is the most common known cause of intellectual disability and developmental disability that can be inherited (passed from one generation to the next).

It is important to note that genes themselves do not cause disease - genetic disorders are caused by mutations that make a gene function improperly. For example, when people say that someone has the cystic fibrosis gene (Read about "Cystic Fibrosis"), they are usually referring to a mutated version of the CFTR gene, which causes the disease. All people, including those without cystic fibrosis, have a version of the CFTR gene.

Abnormalities

People usually have 23 pairs of chromosomes. However, sometimes a person is born with a different number. If a person has an extra chromosome, it is called trisomy. If a person has a missing chromosome, it is called monosomy.

For example, people with Down syndrome have an extra copy of chromosome 21. (Read about "Down Syndrome") This extra copy changes the body's and brain's normal development and causes intellectual and physical problems for the person. Some disorders are caused by having a different number of sex chromosomes. For example, people with Turner syndrome usually have only one sex chromosome, an X. (Read about "Turner Syndrome") Women with Turner syndrome can have problems with growth and development.

Genetic impact on overall health

Virtually every human ailment has some basis in our genes. The Centers for Disease Control and Prevention (CDC) say our genes play a role in nine of the ten leading causes of death in the United States, most notably chronic diseases such as cancer and heart disease. (Read about "Cancer: What It Is" "Coronary Heart Disease") These diseases are partly the result of how genes interact with environmental and behavioral risk factors, such as diet and physical activity. (Read about "Dietary Guidelines" "Getting Started on Fitness")

There are some diseases that are linked to specific mutations that are passed on to us by our parents. A woman's risk of developing breast and/or ovarian cancer is greatly increased if she inherits a certain mutation in the BRCA1 gene or the BRCA2 gene for example. (Read about "Breast Cancer" "The Ovaries") But there are many more diseases that are much more complicated, but also appear to have a genetic component. They are often said to run in families. (Read about "Family Health History")

Family members share genes, behaviors, lifestyles and environments that together may influence their health and their risk of chronic disease. Most people have a family health history of some chronic diseases such as cancer, coronary heart disease, and diabetes (Read about "Diabetes"), or other health conditions such as high blood pressure and high cholesterol. (Read about "Hypertension: High Blood Pressure" "Cholesterol") People who have a close family member with a chronic disease may have a higher risk of developing that disease than those without such a family member, but they are not destined to as with inherited genetic disorders such as Huntington's disease. (Read about "Huntington's Disease")

You can contribute to your health by keeping records of your family health information and sharing this information with your doctor and with other family members.

Our genes are part of who we are. They affect our health, but for the most part they are not destiny. Our health habits, such as diet and exercise, have an impact on our health as well.