Slide show: How genetic disorders are passed from parents to children

Curly hair, dimpled cheeks, brown eyes and athletic ability — these traits run in families. So does the risk of certain diseases.

Almost all diseases have a genetic component. Most diseases — including heart disease and Alzheimer's — result from mutations in multiple genes, combined with environmental factors. Some conditions, such as Down syndrome, are caused by too many or too few genes. And some are caused by a mistake in a single gene.

Single-gene disorders are passed from generation to generation according to straightforward, predictable patterns of inheritance. Understanding these patterns will help you understand the risk of you or your child inheriting one of these disorders. These risks are the same in each pregnancy.

Your body is made up of trillions of cells. Each cell has a center (nucleus) that houses your chromosomes.

Each chromosome is made up of tightly coiled strands of deoxyribonucleic acid (DNA), which consists of four chemical bases — adenine (A), guanine (G), cytosine (C) and thymine (T). Genes are segments of DNA that determine specific traits, such as eye or hair color. You have about 30,000 genes.

A gene mutation is an alteration in your DNA. It can be inherited or acquired during your lifetime, as cells age or are exposed to certain chemicals. Some changes in your genes result in genetic disorders.

In humans, the egg cells and sperm cells have 23 chromosomes. The rest of your cells normally contain 23 pairs of chromosomes. This includes twenty-two pairs of non-sex chromosomes (autosomes) and one pair of sex chromosomes, which determine whether you're male (XY) or female (XX).

You inherit one chromosome in each pair from your father and one from your mother. So, you have two copies of each gene.

Genes may be dominant or recessive. Dominant genes express their trait even if there is only one copy of that gene in the pair. Recessive genes won't show their trait if there is a dominant gene present.

In an autosomal dominant disorder, the mutated gene is a dominant gene located on one of the nonsex chromosomes (autosomes). You only need one mutated gene to be affected by this type of disorder.

A person with an autosomal dominant disorder — in this case, the father — has a 50 percent chance of having an affected child with one mutated gene (dominant gene) and a 50 percent chance of having an unaffected child with two normal genes (recessive genes).

Huntington's disease and Marfan syndrome are autosomal dominant disorders. Mutations to BRCA1 and BRCA2 genes — which have been associated with breast cancer — are also handed down in this pattern.

In an autosomal recessive disorder, the mutated gene is a recessive gene located on one of the autosomes. To have an autosomal recessive disorder — such as cystic fibrosis, sickle cell anemia or phenylketonuria (PKU) — you inherit two mutated genes, one from each parent.

These disorders are usually passed on by two carriers. Their health is rarely affected, but they have one mutated gene (recessive gene) and one normal gene (dominant gene) for the condition. Two carriers have a 25 percent chance of having an unaffected child with two normal genes (left), a 50 percent chance of having an unaffected child who also is a carrier (middle), and a 25 percent chance of having an affected child with two recessive genes (right).

This pattern may also be called sex-linked recessive inheritance. The mutated gene is a recessive gene on the X chromosome.

For a person to have a recessive disorder, it usually requires two copies of the mutated gene. However, males are affected by a single X-linked recessive gene.

A man with an X-linked recessive disorder will pass his normal Y gene to his sons, and none will be affected. He will pass his mutated X gene to his daughters, and all will be carriers of the disease. Their health is rarely affected, but they have one mutated gene and one normal gene for the condition.

Duchenne muscular dystrophy, poor color vision and hemophilia A are X-linked recessive disorders.

Women can also pass down X-linked recessive disorders, such as Duchenne muscular dystrophy, poor color vision and hemophilia A.

A woman who is a carrier of an X-linked recessive disorder has a 25 percent chance of having an unaffected son, a 25 percent chance of having an affected son, a 25 percent chance of having an unaffected daughter and a 25 percent chance of having a daughter who also is a carrier.

In an X-linked dominant disorder, the mutated gene is a dominant gene located on the X chromosome. This pattern may also be called sex-linked dominant inheritance. Only a few, rare disorders follow this pattern, including a rare type of rickets.

A man with an X-linked dominant disorder will pass his normal Y gene to all of his sons and none of them will be affected. He will pass his mutated X gene to all of his daughters and all will be affected.

A woman with an X-linked dominant disorder can pass her mutated X gene to her daughters and her sons. She has a 50 percent chance of having an unaffected son or daughter and a 50 percent chance of having an affected son or daughter. Only a few, rare disorders follow this pattern, including a rare type of rickets.

If you know — or suspect — that you or your partner has a family history of any genetic disorders, a genetic counselor can help assess your risk of having a child with a genetic disorder. This specialized counselor can also help you decide whether or not to have genetic testing. Plus, he or she can explain possible treatments, preventive measures and reproductive options.