Genetic Screening Tests – Autosomal Recessive Diseases

Advancements in medical technology have recently introduced newer and expanding testing capabilities for pregnant women. Actually, many of these tests can be done prior to pregnancy, which is ideal because testing before pregnancy eliminates anxiety associated with waiting for results when a patient is already pregnant. Of course, as we advance science, we also create more to think about and the world of genetics has gotten increasingly complex. We are trying here to educate you and to present the options as simply as possible.

Prior to pregnancy (or during the early part of pregnancy) there is testing available for abnormalities called autosomal recessive disorders. Simply, these abnormalities are related to defects in specific genes and in order for a child to be affected, both parents have to carry the same abnormal gene. When both parents do carry the same abnormal gene, then the chance of having an affected child is one out of four. Cystic fibrosis, Fragile X syndrome and SMA are examples of autosomal recessive disorders.

Cystic fibrosis is the most common autosomal recessive disorder. SMA is less common, but it is the most common cause of infant death in the first 2 years of life. Fragile X is the most common cause of autism.

When one parent tests positive for any of these disorders they are known as a “carrier”. Many of us carry abnormal genes and it is very unlikely that there will ever be a problem. The problems occur when both parents carry the same abnormal gene. Only then does a risk of producing an affected child exist. When both parents have the same abnormal gene, then the chance of having an affected baby is 1 out of 4.

Recently, panels of tests which are capable of testing for over 150 autosomal recessive disorders, including those mentioned above, have become available. Most of these additional disorders are very rare. Unfortunately, we all hear about someone having a baby with a very rare disease. In many instances, this could have been predicted by screening for all of these unusual diseases with a single blood test.

The routine is usually to test one partner. In an Ob/Gyn practice that usually means the female partner. If there are any abnormal genes, then the other partner is tested also, but just for the specific gene abnormality identified in the first one tested. If the couple is unfortunate enough to have both partners with the same abnormality, then genetic counseling is recommended to discuss the implications. It is for this reason that testing is ideal before someone becomes pregnant. That way there is plenty of time to be educated and to have a plan for any future pregnancies. In other words, these tests help assess a couple’s risk of having a child with a specific disease. They are not diagnostic tests to make a diagnosis, which can only be done when there is a specific concern about a specific disease in someone currently pregnant.

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