Each child has two copies of every gene in its body, with one copy coming from each parent. Genes in each pair carry instructions for either dominant or recessive traits. Sometimes the genes are altered in some way, and a medical disorder results.
Autosomal Dominant Inheritance
If a parent has the gene for an autosomal dominant condition, there is a 50 percent chance (a one-in-two chance) that each child will have the same condition. Dominant disorders often are quite variable, with symptoms ranging from none to severe.
Some conditions passed on through autosomal dominant inheritance are:
- Familial high cholesterol
- Huntington disease, a progressive nervous system disorder
- Some forms of glaucoma, which cause blindness if untreated
- Polydactyly: extra fingers or toes
- Marfan syndrome, which affects connective tissue (connective tissue supports and connects structures of the body; tendons, ligaments, cartilage and bone are examples of connective tissue)
Autosomal Recessive Inheritance
If both parents are carriers of the same recessive gene that can cause a birth defect, there is a one-in-four chance that each of their children will inherit the problem. If only one parent passes on the gene for the disorder, the normal gene received from the other parent will prevent expression of the condition.
Autosomal recessive disorders often are severe and may lead to an earlier death. Some conditions passed on through autosomal recessive inheritance are:
- Sickle cell disease, a blood disorder that mainly affects people of African-American and Hispanic descent
- Tay-Sachs disease, which causes mental retardation and death mainly in people of Eastern European Jewish or French Canadian heritage
- Cystic fibrosis, a disorder of the lungs and digestive system affecting mainly people of Northern European Caucasian descent
- Phenylketonuria (PKU), a metabolic disorder primarily affecting Caucasians
X-Linked Recessive Inheritance
The X and Y chromosomes are the sex-determining chromosomes. Normal females have two Xs, and normal males have one X and one Y. A disorder stemming from an abnormal gene on one of the X chromosomes is called an X-linked, or sex-linked, disorder.
An apparently normal mother with an abnormal gene on one of her X chromosomes has a 50 percent (one-in-two) chance of passing it to her child. A female who inherits one X with a gene for a sex-linked disorder usually has no symptoms of the disease, because she has, in effect, a spare X with a normal copy of the same gene. However, males who inherit an X chromosome with a gene for a sex-linked disease have no second X to fall back on and, therefore, have the disease.
Some conditions passed on through X-linked recessive inheritance are:
- Hemophilia, in which the blood lacks a substance needed for clotting
- Red-green color blindness
- Duchenne muscular dystrophy, which causes muscle weakness and death
