Slide show: How genetic disorders are inherited
Your genes, chromosomes and DNA
Your body is made up of trillions of cells. Each cell has a core structure (nucleus) that contains your chromosomes.
Each chromosome is made up of tightly coiled strands of deoxyribonucleic acid (DNA). Genes are segments of DNA that determine specific traits, such as eye or hair color. You have more than 20,000 genes. You have two copies of each gene — one inherited from each of your parents.
A variant is a change in your DNA. Some variants are benign — they're not associated with health problems. Other variants are pathogenic — they're associated with a genetic or health condition. Pathogenic variants are sometimes called mutations.
Many types of pathogenic variants cause a change in or damage to a gene. These abnormal genes (gene mutations) can be inherited or acquired. Acquired gene mutations may be associated with environmental exposures. We often don't understand the specific exposures that result in an acquired gene mutation.
Most cells in your body normally contain 46 chromosomes, organized into 23 pairs. In each of these 23 pairs, you've inherited one chromosome from your father and one from your mother.
Of these 23 pairs, 22 pairs are nonsex chromosomes (autosomes) and the remaining pair is made up of sex chromosomes, which determine whether you are male (XY) or female (XX). This illustration shows chromosomes from a male cell.
Disorders may have a dominant or recessive inheritance pattern. Dominant inheritance means that only one copy of the gene — from either the mother or the father — needs to have a mutation (pathogenic variant) for the trait or disease to be expressed. Recessive inheritance means the trait or disease will only occur if both copies of the gene — one from the mother and one from the father — are abnormal (mutated).
Autosomal dominant inheritance pattern
In an autosomal dominant disorder, the abnormal (mutated) gene is located on one of the first 22 pairs of chromosomes (autosomes). You need only one copy of the abnormal gene to be affected by this type of disorder.
A person with an autosomal dominant disorder — in this case, the father — has a 50% chance of passing on the abnormal gene (and having an affected child) and a 50% chance of passing on the normal copy of the gene (and having an unaffected child) with every pregnancy.
Huntington's disease and Marfan syndrome are two examples of autosomal dominant disorders. Mutations to BRCA1 and BRCA2 genes — which have been associated with breast cancer — also are transmitted in this pattern.
Autosomal recessive inheritance pattern
In an autosomal recessive disorder, the abnormal (mutated) recessive gene is located on one of the nonsex chromosomes (autosomes). To inherit an autosomal recessive disorder — such as cystic fibrosis, sickle cell anemia or phenylketonuria (PKU) — both parents must be carriers. The child inherits two copies of the abnormal gene — one from each parent.
A carrier is an unaffected individual who has one copy of the abnormal gene and the other copy is normal. With each pregnancy, two carriers have a 25% chance of having an unaffected child with two normal copies of a gene (left), a 50% chance of having an unaffected child who also is a carrier (middle), and a 25% chance of having an affected child with two abnormal copies of the gene (right).
X-linked recessive inheritance pattern with affected father
In an X-linked recessive inheritance pattern, the abnormal (mutated) gene is on the X chromosome. Duchenne muscular dystrophy, some types of colorblindness and hemophilia A are examples of X-linked recessive disorders.
A man with an X-linked recessive disorder will pass his unaffected Y chromosome to his sons, and none will be affected. He will pass his X chromosome (with the abnormal gene) to his daughters, and all will be carriers of the disease. His daughters may have no symptoms or only mild features of the disorder, and they may pass the abnormal gene to their children.
X-linked recessive inheritance pattern with carrier mother
Female carriers of an X-linked recessive disorder often have no symptoms, though some have mild symptoms of the disorder. This is because female carriers have one normal copy of the gene and one abnormal (mutated) copy. The normal copy typically can compensate for the abnormal copy.
Males have only one X chromosome and will usually be affected if there is an abnormal copy of a gene on the X-chromosome.
A woman who is a carrier of an X-linked recessive disorder has a 25% chance of each of the following results with each pregnancy:
- Having an unaffected son
- Having an unaffected daughter
- Having an unaffected daughter who also is a carrier
- Having an affected son
If you know — or suspect — that you or your partner has a family history of any genetic disorder, a genetics professional can help assess your genetic risk. Genetic counseling can also help you decide whether to have genetic testing and learn about treatments, preventive measures and reproductive options.
Last Updated Feb 15, 2020