Carrier testing is a type of

genetic testing Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, or ...

that is used to determine if a person is a carrier for specific autosomal recessive diseases. This kind of testing is used most often by couples who are considering becoming pregnant to determine the risks of their child inheriting one of these genetic disorders.

Background

Gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

s come in pairs; one from the mother and one from the father. A carrier is a person who inherited one abnormal gene from one of their parents. Carriers often show no symptoms of the genetic disorder that they carry an abnormal gene for. Usually the only time a person finds out that they are a carrier for a specific genetic disorder is when they have an affected child. For these kinds of genetic diseases to be present in a person, two copies of the abnormal gene are needed. This means that both of the parents have to be a carrier for the child to inherit the disease.

Reasons for testing

The most common reason for carrier testing to allow future parents to find out if they are a carrier for a genetic disorder. If the parents are a carrier for a genetic disorder they can know ahead of time what their probability is of having an affected child. Carrier testing can be done before or during the pregnancy. Many of these genetic disease are fatal at a young age which oftentimes encourages parents to get tested.

Autosomal recessive disorders/diseases

There are hundreds of recessive genetic disorders, most of which are very rare. Certain genetic disorders tend to be more common in people of a particular

ethnicity An ethnicity or ethnic group is a group of people with shared attributes, which they Collective consciousness, collectively believe to have, and long-term endogamy. Ethnicities share attributes like language, culture, common sets of ancestry, ...

. For example, people of

African American African Americans, also known as Black Americans and formerly also called Afro-Americans, are an Race and ethnicity in the United States, American racial and ethnic group that consists of Americans who have total or partial ancestry from an ...

ethnicity have a much higher likelihood of being a carrier for the autosomal recessive disorder called

sickle cell anemia Sickle cell disease (SCD), also simply called sickle cell, is a group of inherited haemoglobin-related blood disorders. The most common type is known as sickle cell anemia. Sickle cell anemia results in an abnormality in the oxygen-carrying ...

. People of one ethnicity in particular,

Ashkenazi Jew Ashkenazi Jews ( ; also known as Ashkenazic Jews or Ashkenazim) form a distinct subgroup of the Jewish diaspora, that Ethnogenesis, emerged in the Holy Roman Empire around the end of the first millennium Common era, CE. They traditionally spe ...

ish, have a tendency to be carriers for a wide variety of recessive genetic disorders. There are also several recessive disorders that are present in all ethnicities. This list includes:

cystic fibrosis Cystic fibrosis (CF) is a genetic disorder inherited in an autosomal recessive manner that impairs the normal clearance of Sputum, mucus from the lungs, which facilitates the colonization and infection of the lungs by bacteria, notably ''Staphy ...

fragile X syndrome Fragile X syndrome (FXS) is a genetic neurodevelopmental disorder. The average IQ in males with FXS is under 55, while affected females tend to be in the borderline to normal range, typically around 70–85. Physical features may include a lo ...

, and

spinal muscular atrophy Spinal muscular atrophy (SMA) is a rare neuromuscular disorder that results in the loss of motor neurons and progressive muscle wasting. It is usually diagnosed in infancy or early childhood and if left untreated it is the most common geneti ...

Testing methods

Carrier testing is most often done through a simple

blood test A blood test is a medical laboratory, laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a hypodermic needle, or via fingerprick. Multiple tests for specific blood components, such as a glucose ...

. The results from these tests are usually available anywhere between two and eight weeks depending on where the testing is conducted. Some tests may screen for one or a few genetic conditions, and other tests may screen for hundreds. For example, in Australia, multiple companies offer carrier screening tests for the carrier status of three genes, whereas an extended screen is available that looks for variations in 552 genes. An alternative method of testing, available for some conditions, analyzes

gene product A gene product is the biochemical material, either RNA or protein, resulting from the expression of a gene. A measurement of the amount of gene product is sometimes used to infer how active a gene is. Abnormal amounts of gene product can be corre ...

s that are usually present in a person that keeps the genetic disorder from occurring. An affected person with the disorder would have close to a hundred percent reduction of the gene products compared to an unaffected person. A carrier would only have a fifty percent reduction in those gene products.

Risks involved

The physical risks for getting this kind of genetic testing done are very minimal. The most common requirement is a blood sample. The emotional risks on the other hand are great. When a person finds out they are a carrier for a specific genetic disorder, dealing with that can be very difficult. In many cases, people who find out they are carries can become angered or even enraged that they carry a genetic defect that could be passed on to their child. These results can play a role in determining if a couple will have a child together. If both parents are carriers for the same genetic disorder, there is a twenty-five percent chance that any child they have could be affected. When a person finds out they are a carrier, they are always encouraged to talk to a genetic counselor. If both partners are carriers for the same genetic disease, the choice to have a child together can become much more difficult. IVF with

preimplantation genetic diagnosis Preimplantation genetic diagnosis (PGD or PIGD) is the genetic profiling of embryos prior to implantation (as a form of embryo profiling), and sometimes even of oocytes prior to fertilization. PGD is considered in a similar fashion to prenatal ...

may be considered, to remove the risk of having an affected child.

References