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Genes are the basic units of heredity. In everyday terms, we often think of genes as being instructions for a certain 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 its own function in the body.
For example, a gene may have the instructions to make a protein called an enzyme, which is needed 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 alike in males and females and are called autosomes. The last pair, the sex chromosomes, differs in males and females. Males have one X and one Y chromosome, and females have two X chromosomes.
Genetic disorders arise when one or both copies of a certain gene have a genetic change that leads to a protein product that does not work in the body like it should. Sometimes these genetic changes are called mutations. When a change in only one copy of a gene pair causes a genetic disorder, the disorder is called dominant.
Someone who has a gene for a dominant disorder is usually affected by the disorder. Each parent passes only one copy of every gene to their child. So if either the mother or father has a dominant genetic condition, there is a 50% chance that their child will have the condition. The same risk applies to each child the couple has, regardless of whether older children have had the disorder.
A dominant disorder sometimes appears in a child whose parents do not have the disorder. This occurs when a new mutation has arisen in a gene. This means the genetic change 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 called recessive. In many cases, when a single copy of a gene undergoes a mutation, there is no evidence that it happened. In these cases, the disorder is recessive since the normal copy of the gene appears to offset the error in the mutated gene. However, when both copies of the gene are abnormal, the disorder is present. In order for a child 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 healthy, 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, like the parents). Often there 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 of his sons. Examples of genetic conditions that are inherited in this manner are Hunter syndrome (MPS II) and Hemophilia A.