Genetic Conditions: How Do They Run in Families?

A genetic condition is one that happens when a gene changes. Some genetic conditions can be inherited (passed from parents to their children).

What Is a Gene?

Genes are sections of DNA (deoxyribonucleic acid) that are inside every human cell. Genes carry information that passes from one generation to the next. For example, genes are why a child has the same eye color or hair color as a parent and why some health conditions run in families.

Genes are found on small spaghetti-like structures called chromosomes (KRO-moh-somes). Most people have 23 pairs. There are two kinds of chromosomes:

• Autosomes: These 22 pairs of chromosomes are the same in males and females.
• Sex chromosomes: This 23rd pair determines the sex of a baby. Most females have two X chromosomes (XX), one from each parent. Most males have one X chromosome and one Y chromosome (XY). The X is from the mother, and the Y is from the father.

How Do Genes Pass From Parent to Child?

To make a baby, an egg from the mother and a sperm from the father come together. The egg and sperm each have half a set of chromosomes. Together, they give the baby a full set of chromosomes. So, half the baby’s genes come from the mother and half come from the father.

If one gene has a change (mutation), it can cause a condition called a single gene disorder. These disorders can pass from parent to child in different ways called inheritance patterns.

What Are Some Types of Inheritance Patterns?

Inheritance patterns that pass changed genes from parent to child include:

• Autosomal: The changed gene is on an autosomal chromosome.
• X-linked: The changed gene is on the X chromosome.
• Dominant: One changed gene from a parent is needed for symptoms of a genetic condition to happen.
• Recessive: Two changed genes (one from each parent) are needed for symptoms of a genetic condition to happen.

What Is an Autosomal Dominant Inheritance Pattern?

In an autosomal dominant inheritance pattern, the child gets a changed gene on an autosome chromosome from one parent and a healthy gene from the other parent. Both the child and the parent with the changed gene usually have symptoms of the genetic condition.

Autosomal dominant genetic conditions include:

• Marfan syndrome
• achondroplasia

What Is an Autosomal Recessive Inheritance Pattern?

With this type of inheritance pattern, a child gets a changed gene on an autosome chromosome from each parent. The child has symptoms of the genetic condition, but the parents don’t, so they may not know they have it. The parents are called carriers. Carriers don’t usually have symptoms of the condition, but can pass the changed gene to their kids.

Autosomal recessive genetic conditions include:

• sickle cell disease
• cystic fibrosis
• phenylketonuria (PKU)
• Tay-Sachs disease

What Is an X-Linked Dominant Inheritance Pattern?

With an X-linked dominant inheritance pattern, a child gets a changed gene on the X chromosome from one parent and a healthy gene from the other parent. Both the child and the parent with the changed gene on the X chromosome usually have symptoms of the genetic condition.

X-linked dominant genetic conditions include:

• some kinds of rickets
• fragile X syndrome (a condition that can cause problems with thinking and learning)

What Is an X-Linked Recessive Inheritance Pattern?

With an X-linked recessive inheritance pattern, a male (who has an X and a Y chromosome) gets a changed gene on the X chromosome from his mother. He inherits the father’s Y chromosome, so he can’t get a changed X-linked gene from him. Males need only one changed gene to show symptoms. The mother (unless she has two changed genes) doesn’t have symptoms. The father doesn’t have the changed gene, so he doesn’t show symptoms.

A female (who has two X chromosomes) gets a changed gene on the X chromosome from each parent. She has symptoms of the genetic condition, but the parents don’t and might not know they have it.

X-linked recessive genetic conditions include:

• hemophilia
• Duchenne muscular dystrophy
• some kinds of colorblindness (not being able to see certain colors)

Can I Prevent Passing a Genetic Condition to My Child?

You can’t know exactly which genes your child will get, but doctors can look at your genes with genetic testing. This can show if you have a condition, are at risk for it, or are a carrier for it.

A genetic counselor can help you decide what tests to get and understand what the results mean.