Recessive disorders are a direct result of the genes passed down to a child from the parents. Some recessive conditions may cause no noticeable problems while others are fatal.
A recessive disorder is a disease or trait that is caused by genes encoding improper information. In many situations, an individual may never realize anything is wrong if the person has one functioning gene along with the abnormal gene. The actions of the working gene will compensate for the fact that one of the genes contains “incorrect” information. These people are known as carriers and it is when two carriers have a child that a recessive condition has a chance to appear. Each disorder is unique with the severity ranging from mild to fatal. Some recessive genes are so fatal that the inheritance of both genes will cause the fetus to never form. A great example of this is the Manx cat. If a kitten inherits two “manx” genes, it will never be born.
The appearance of a recessive disorder is dependent on the type of inheritance pattern it follows. The most common are autosomal recessive inheritance patterns. This is when the abnormal gene is present on any of the 22 non-sex chromosomes. Because genes are present in pairs, each parent will give a child one gene. When both parents are carriers, they each have a 50% chance of passing on the dysfunctional gene. This means that there is a 1 in 4 chance that a child will get a dysfunctional gene from both parents. To sum it up, autosomal inheritance means:
- 25% chance that the infant will have two normal genes
- 50% chance that the infant will be a carrier with one normal and one atypical gene
- 25% chance that the infant will have two atypical genes and therefore exhibit the disease
- Keep in mind that the percentages are applied to each pregnancy separately.
Another type of inheritance pattern that will result in a recessive disorder is sex-linked recessive. This is when the problem gene is located on the sex chromosome (chromosome pair 23). The gene is either on the X or Y gene. Normal males have a XY genetic make-up while females are XX. The chance of having a child afflicted with a disorder linked to the sex genes is dependent on which gene it is on and whether or not the other parent is a carrier. In essence, the principle is the same. A female will need to have two abnormal X’s to be sick while a male will need just one abnormal X or one abnormal Y because there is no duplicate gene to compensate.
The only way to prevent having a child with a specific recessive disorder is to not have children with another carrier. Even though with autosomal inheritance there is a 25% chance of having a child with the disease, it is possible to have many healthy children. Individuals with known disorders in their family history may want to consider getting genetic counseling. These counselors will look at the family medical history of both parents in order to determine the odds of having a child with a particular ailment. Nothing can ever be 100% guaranteed.