The carrier screening is a test that can be done as a normal part of obstetric care. This test, which can be done before you conceive or during pregnancy, can tell you if you and your partner are at risk of passing along certain diseases to your children that you may not have yourself.
What does it mean to be a "Carrier"?
Just about everyone carries a gene for at least one disorder even if it has never shown up in your family history. If you have a mutation in just one set of genes, you are what is known as a “carrier”: You are carrying the genes for a disorder but have no signs of the disease.
Rarely, a condition (like Huntington’s disease) can be caused by a mutation in just one set of DNA from one parent. But in most cases, including cystic fibrosis and sickle cell disease it takes a mutation in both sets of DNA, one from Mom and one from Dad, for a child to actually be affected. Therefore, these diseases are rare.
Carrier Screening is designed to identify the risks
A carrier screening tests whether you or your partner is a carrier for certain genetic diseases. It is a swab test, which requires a quick swab on the inside of your cheek. If both you and your partner are carriers, and you both pass the mutation-carrying DNA along to a baby, the baby could end up with a full-fledged version of the disease. This test determines the likelihood of this happening.
Who is a candidate for this testing?
Parents can be tested for some of disorders before (which is preferable) or during pregnancy. In almost all cases, testing is recommended for one parent, whereas testing the second parent only becomes necessary if the first tests positive.
In the past, the American Congress of Obstetricians and Gynecologists (ACOG) only recommended carrier screenings to parents of certain ethnic or geographic backgrounds considered more at risk of specific disorders. However, some conditions are not limited to one ethnicity. Today’s environment we have many people are of mixed ethnic backgrounds, which means it can be hard to make recommendations based on an ethnic or geographic basis. Although they are still very rare, some conditions are common enough that practitioners should offer to screen for them in every patient.
Because some people are more likely to carry specific gene mutations, your doctor may recommend a number of other screenings based on your family heritage and/or medical history:
- Eastern and Central Eastern (Ashkenazi) Jewish descent: Tay-Sachs disease, Canavan disease and familial dysautonomia; there are several other disorders some practitioners screen for including Bloom syndrome, Maple syrup urine disease and Niemann-Pick disease, among others
- French-Canadian or Cajun: Tay-Sachs disease
- Ovarian insufficiency syndrome: Fragile X syndrome; the screening should also be offered to women with a family history of fragile X-related disorders
- Other family history of disease: Anyone who has a family history of genetic diseases such as a cousin who had Tay-Sachs disease, for instance, should be screened for those diseases as well, as they’re more likely to be a carrier
What are the odds of our baby having a disease if we test positive?
Even if both you and your partner test positive as carriers of the same gene mutation, there’s still only a 1 in 4 (or 25 percent) chance that your baby will have the disease. That’s because each of you has two sets of DNA. Since you are carriers and do not actually have the disease, that means you each have a second, healthy copy. If the baby inherits the healthy copy from one or both of you, he will not have the disease (although your child may be a carrier).
Is there anything we can do if we test positive?
The most powerful time to get information from carrier screening is before you’re pregnant. If you and your partner do test positive as carriers of a disease, you may choose to work with fertility doctors to discuss your options.
If you get a carrier screening done after you are already pregnant, a positive result can mean more tests to see whether your baby is affected. A positive result on one of these tests can be valuable to help you think ahead and plan financially. It could also lead to early treatments for the disease before the baby is born as well as arrange for special care during and after birth.
And since these disorders do tend to run in families, ACOG suggests that you should inform relatives of positive results so they can decide whether they would like to be screened as well.
Even if you don’t talk to a genetic counselor before proceeding with carrier screening, many health care practitioners will refer you to one to help you interpret your results and decide what to do with any information you obtain. As more and more tests become available, understanding what they mean can be confusing and these counselors are trained in helping you sort through the available information to make sense of it.
We have 46 chromosomes that come in pairs (23 pairs). One of each pair is inherited from you mother and the other from your father. The last set of chromosomes are called sex chromosomes and are paired as XX (female) or XY(male). There are five basic modes of inheritance for single-gene diseases to exist: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and mitochondrial.
Autosomal Dominant: Only one copy of a gene has a mutation/change in each cell that leads to a genetic condition. There is a 50% risk to pass on the condition to your children and typically each affected person has one affected parent.
Autosomal Recessive: Both copies of a gene has a mutation/change for an individual to be affected with the genetic condition. Parents are usually unaffected but each “carry” a mutation in one of their two copies of the gene. If both parents are carriers of the same genetic condition, there is a 1 in 4 or 25% chance to have an affected child.
X-linked Recessive: A mutation/change is located on the X chromosome. Typically, only males are affected if they have the mutation since they have only one x-chromosome. Females with the mutation are typically unaffected because they have two x-chromosomes, but are considered “carriers”. If a woman is a carrier for an X-linked condition, there is a 50% chance to her son to be affected, and a 50% chance for her daughters to be carriers. If a male is affected then ALL his daughters will be carriers and ALL of his sons will be healthy and not carriers.
Rarely, females can inherit an X-linked recessive disorder. If both parents are affected, ALL of their children will be affected. If the father is affected and the mother is a carrier, 50% of their children will be affected (boys or girls), while the other 50% will be healthy boys or carrier girls.
X-linked Dominant: Similar to X-linked recessive, a mutation/change is located on the X chromosome. However, with X-linked dominant mutations, you only need one mutation to be affected. Both males and females can be affected, although males may be more severely affected or may not survive. When a female is affected, each pregnancy will have a 50% chance for offspring to inherit the disease allele. When a male is affected, all his daughters will be affected, but none of his sons will be affected.
For most genes, everyone inherits two copies: one from mom and one from dad. A carrier is an individual who has one mutated copy and one normal copy of the same gene. Carriers typically do not have signs or symptoms of a genetic disorder.
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