A new prenatal test is now available. It is called prenatal carrier screening and tests parents for possible genetic diseases that they could inherit to their offsprings. Testing the parents is done by a simple blood draw and we can test either parent’s carrier status of serious conditions that they could pass to their baby. Ideally, this test can be performed before the desired pregnancy but also has a place for an ongoing pregnancy as well. With this test we can check for more than 400 possible genetic conditions.
What is disease carrier testing (carrier screening)?
It is quite common for humans to be a carrier of at least one genetic disease. Carriers (only one of the two copies of genes is defective) are usually healthy. For most of the future parents, there is no family history of a genetic disease, nor some knowledge what it would mean for their child to inherit the disease. Carrier screening test is a new kind of genetic test that permits you to discover the risk for your child to inherit a genetic disease passed by you.
When this test is done before conception, carrier screening can provide valuable information and the opportunity to pursuit your options about the appropriate way of creating a family. When the carrier screening test is performed during pregnancy it can also the be beneficial: It may help couples decide if they wish to have a diagnostic test (like amniocentesis) but can also help them prepare psychologically, emotionally, medically and financially for the birth of a baby carrying a genetic disease.
What medical conditions can we detect?
Depending on the depth of the analysis, we can control for the 22 commonest genetic syndromes or diseases or expand to more than 400 medical conditions that can be inherited. You can see in the following table, the conditions we assess with the 22 diseases carrier screening test.
| Disease | Gene | Description |
| Alpha Thalassemia | HBA1/HBA2 | |
| Beta-Hemoglobinopathies | HBB | |
| Deltabeta Thalassemia | HBD,HBB | |
| Sickle-Cell Disease | HBB G | |
| Cystic Fibrosis | CFTR | 100% analysis |
| Spinal Muscular Atrophy | SMN1 | |
| Bloom Syndrome | BLM | |
| Canavan Disease | ASPA | |
| Non-Syndromic Hearing Loss (GJB2-related) | GJB2 | |
| Non-Syndromic Hearing Loss (GJB6-related) | GJB6 | |
| Duchenne/Becker Muscular Dystrophy | DMD | |
| Familial Dysautonomia | IKBKAP (ELP1) | |
| Fanconi Anemia, Group C | FANCC | |
| Fragile X Syndrome, X- linked | FMR1 | |
| Galactosemia | GALT | |
| Gaucher Disease | GBA | |
| Medium Chain Acyl-CoA Dehydrogenase Deficiency | ACADM | |
| Mucolipidosis, Type IV | MCOLN1 | |
| Niemann-Pick Disease, Types A/B | SMPD1 | |
| Smith-Lemli-Opitz Syndrome | DHCR7 | |
| Phenylalanine Hydroxylase Deficiency | PAH | |
| Tay-Sachs Disease (DNA only) | HEXA |
How are genetic diseases inherited?
Generally, there are three ways:
1) Genetic diseases caused by a dominant gene: There has to be only one defective gene in order for the disease to appear (not examined by the carrier screening test)
2) Genetic diseases caused by a recessive gene: It takes two defective genes to be present in order for the disease to appear (these diseases are examined by the carrier screening test)
3) Genetic diseases linked to chromosome X (sex chromosome): They are inherited by the mother that has one defective gene (those diseases are also been tested by the carrier screening test)
Genetic diseases caused by a recessive gene
When the gene causing the disease is recessive, a couple may have an offspring with the disease when both the mother and the father are carriers. That is, they should simultaneously carry one defective gene, for the same disease. For each pregnancy, there is a 25% probability to have a child with the disease and a 75% probability for their child to be healthy.
Genetic diseases linked to chromosome X (sex chromosome)
Sex chromosomes are two and named Χ and Υ. Male gender has one Y chromosome and one X chromosome. Female gender has two chromosomes X. Some genetic diseases are caused by mutations in genes located on the X chromosome. Women may just be carriers of the disease (but absolutely healthy) while men can either be healthy or having the disease (there is not the intermediate state of the carrier).
For this kind of genetic diseases, it takes only for the mother to be a carrier for the disease to be inherited to the offspring. More precisely, for each pregnancy, there is a 50% probability for a boy to inherit the disease. If the mother has a girl, then there is a 25% probability for the daughter to be carrier. Irrespective of the offspring’s gender, there is a 75% probability of the child (boy or girl) to be healthy.
I had the carrier screening test and I am a carrier of a genetic disease. What’s next?
Couples that plan a future pregnancy
If we find out that you are a carrier for a gene causing a specific disease, then we could check your partner for the same gene. We need to discuss what is the appropriate test. If we discover that your partner also carries the defective gene for the same disease, then we should discuss your options about having a family.
If the disease is sex related (linked to chromosome X) then we don’t usually have to test your partner as it only takes for the mother to be a carrier in order for the disease to be inherited.
We had the test and it seems that we are both carriers of the same genetic disease. What are our options for a future family?
Knowing this information, future parents can consider their fertility options. Couples that may have a chance of passing a recessive or sex linked genetic disease may decide to pursue one of the following choices:
1) Natural conception with (or without, if chosen) further diagnostic test i.e chorionic villous biopsy (CVS) or amniocentesis.
2) Preimplantation Genetic Test (PGT) following an IVF protocol, in order to check the fertilised fetuses. Only those found free of the disease are, subsequently, implanted in the womb.
3) Use of donor egg or sperm
4) Adoption
Couples already pregnant
If we find out that you are a carrier for a gene causing a specific disease, then we could check your partner for the same gene. We need to discuss what is the appropriate test. If we discover that your partner also carries the defective gene for the same disease, then we should discuss your options about having a family.
If the disease is sex related (linked to chromosome X) then testing your partner is not usually recommended, as it only takes for the mother to be a carrier in order for the disease to be inherited.
Carrier screening test during pregnancy can have life-changing beneficial effects. It can help couples decide whether they wish to further have a diagnostic test like chorionic villous biopsy (CVS) or amniocentesis and, if needed, have the appropriate time to prepare themselves emotionally, medically and financially for the birth of a baby with a genetic disease.