Genes are the fundamental units of heredity. In everyday terms, we often think of genes as being instructions for a particular trait, such as hair or eye color. Genes are made of a chemical called deoxyribonucleic acid (DNA). Each gene has the chemical instructions for making a specific protein, and each protein has a particular function in the body.
For example, a gene may have the instructions to make a protein called an enzyme, which is necessary for an important biochemical process in the body.
Individual genes are packaged into structures called chromosomes. Humans have a total of 46 chromosomes which are located in the center of every cell of the body. We receive 23 chromosomes (half our genetic information) from our mothers and 23 chromosomes from our fathers. Of those 23 pairs, 22 of them are identical in men and women and are called autosomes. The last pair, the sex chromosomes, differs in men and women. Men have one X and one Y chromosome, and women have two X chromosomes.
Autosomal Dominant Inheritance
Genetic disorders arise when one or both copies of a specific gene have undergone an alteration known as a mutation. When an alteration in only one copy of a gene pair causes a genetic disorder, the disorder is referred to as dominant.
Someone who possesses a gene for a dominant disorder is usually affected by the disorder. Since each parent passes only one copy of every gene to a child, a child conceived by an affected individual has a 50% chance of inheriting the abnormal copy of the gene (and inheriting the disorder), and a 50% chance of inheriting the normal copy of the gene (and not inheriting the disorder).
The same risk applies to each child a couple has, regardless of whether older children have had the disorder. A dominant disorder sometimes appears in an individual whose parents do not have the disorder. This occurs when a new mutation has arisen in a gene. This means the alteration has occurred for the first time in the sperm or egg that resulted in the conception of the child, rather than being inherited from one of the parents.
Dominant disorders can occur in multiple generations in a family and do not skip a generation. They affect men and women equally. There can be multiple affected family members in one generation. Except in cases of a new mutation, every affected individual has an affected parent. An example of a condition inherited in this fashion is neurofibromatosis type 1.
Autosomal Recessive Inheritance
When alterations in both copies of a gene are required for the disorder to be present, the disorder is considered recessive. In many cases, when a single copy of a gene undergoes a mutation, there is no evidence of that alteration. In these instances, the disorder is considered recessive since the normal copy of the gene appears to dominate the altered one. However, when both copies of the gene are abnormal, the disorder becomes apparent. In order for an individual to have two abnormal copies of a gene, an abnormal copy of the gene must be inherited from both parents.
It is expected that both parents of an affected child are entirely normal, even though they both carry the altered gene. Couples who have had a child affected with a recessive disorder (or who are both known to be carriers of a recessive gene by testing) have a 25% chance that any future child will have the disorder, a 25% chance that the child will inherit both normal copies of the gene, and a 50% chance that the child will inherit a normal copy of the gene from one parent, and an abnormal copy of the gene from the other (will be a carrier). There typically is not a family history in autosomal recessive disorders, since these conditions require both parents to be carriers. Carrier parents of an affected child are expected to be normal and healthy. Phenylketonuria (PKU) is an example of a genetic condition inherited in this way.
X-linked Dominant Inheritance
This occurs when there is a mutation in a gene that is on the X chromosome. Disorders that are inherited in this way can affect both males and females, but are more commonly seen in females. A father cannot pass along the trait to his son because boys always inherit the Y chromosome from their fathers. However, 100% of daughters of an affected father would have the condition because girls always inherit the X chromosome from their father. In the instances in which the mother is affected with the trait, there is a 50% chance that any of her children will be affected by the disorder. An example of a genetic condition that is inherited this way is Fragile X syndrome.
X-linked Recessive Inheritance
Disorders of this type of inheritance also occur when there is a mutation in a gene that is on the X chromosome. Usually only males are affected with this type of disorder because females have 2 X-chromosomes, and the altered copy of the gene is recessive to the normal copy. As in the case of X-linked dominant inheritance, a father cannot pass along the trait to any sons that he may have. Examples of genetic conditions that are inherited in this manner are Hunter syndrome (MPS II) and Fabry disease.