Lesson Explainer: Medical and Genetic Screening Biology

In this explainer, we will learn how to recall some examples of diseases that can be genetically or medically screened for, and explain why genetic screening is offered to certain individuals.

One of the most useful applications for understanding our genetics is being able to treat serious human illnesses. Knowledge about our genetic makeup means we also have data on the genetic code for the illnesses we carry and can potentially pass down to our offspring. This ability to examine our genes for the presence of an illness can be very helpful for families, especially those that are planning their parenthood experiences.

In reproduction, half of the chromosomes of the offspring are passed down or inherited from the biological father, while the other half are inherited from the biological mother, as shown in Figure 1. This means that any illness or sickness that is coded for within the genes of our biological parents has the potential to also be in our genes!

In order to discover the potential health issues that are related to our genes, a screening of our genetic code can be performed. Genetic screening is a type of medical testing that can help identify differences in a person’s genetic makeup to predict or rule out an inherited genetic disorder. Genetic disorders are illnesses caused by differences in a person’s genetic makeup. Since genetic disorders involve changes or alterations in the genetic code, it is possible for a genetic disorder to be passed down from the biological parents to their offspring, as shown in Figure 2. Genetic disorders often involve linkage to one of the sex chromosomes or require recessive alleles to be inherited from both biological parents.

Genetic or medical screening can be performed for a number of reasons. The most common are the following:

1. Advanced age of the parents of an unborn offspring
2. Learning if a genetic disorder that runs in the family is present before symptoms are seen
3. Determining the likelihood that a current or future pregnancy will pass on a genetic disorder
4. Testing for diseases that are sexually transmitted, like human immunodeficiency virus or hepatitis, before trying to have children

Sexually transmitted infections can be very dangerous to developing offspring, and the process of birth can sometimes even pass the infection from mother to baby. So, medical screening is particularly important for preventing the passage of these infections from mother to baby.

Genetic screening can be performed at any stage of life, from pregnancy to adulthood. Most genetic screenings are requested or used during planning conception and pregnancy. This is because genetic disorders are frequently discussed when trying to conceive, which is another word that means “trying to become pregnant.” Genetic disorders can be tested for prepregnancy, during pregnancy, and immediately after birth. The screenings are typically done using a blood or saliva sample and results are ready in a few days.

When screening for genetic disorders, the following are the five most common tests used:

• Carrier testing
• Preimplantation testing
• Prenatal testing
• Newborn testing
• Presymptomatic testing

Let’s go over each of these genetic tests and the stage of pregnancy when the genetic tests are routinely requested.

In the prepregnancy stage, parents who wish to have offspring may consider genetic screening. For example, if one or more parents belong to a high-risk category, like a family history of illness or advanced age for either parent, a genetic screening can help ensure the offspring are healthy. Similarly, in certain cultures, marriage between relatives may necessitate the need for genetic screening. People who have a family history of an inherited disease or who share a close genetic relationship, such as belonging to certain ethnic groups, may have a comparatively higher risk of specific inherited diseases. For couples where individuals share close genetic makeup (i.e., the variety of gene versions in a person), this can lead to an increase in diseases that require two recessive versions of the gene to be inherited. So, in order to ensure the offspring from these marriages are healthy, genetic screening may be requested.

For high-risk parents or those who are relatives, the genetic test used in genetic screening in the prepregnancy stage is carrier testing. If an individual knows their family history, has no symptoms, and is uncertain about being a carrier of a lethal gene, they may request genetic screening before trying to conceive. Carrier testing is a type of genetic screening for people with a history of a genetic disease in the family, but who do not have symptoms. Since the individual may not have symptoms, it can be unclear if they carry the disease.

People who are trying to conceive often use carrier testing to determine if one or more people contributing genetic material to the pregnancy carry a recessive gene for a disease. Carrier testing is excellent for these couples because it can help determine if either person is a carrier of the alleles for an autosomal recessive disease. The results of carrier testing can help couples plan for financial and emotional impacts of living with a genetic disorder. If the two individuals trying to conceive both carry the autosomal recessive genes, there is a higher probability that their offspring will be affected by the inherited disorder. The inheritance that leads future offspring to carry both recessive alleles of an autosomal recessive disease is shown in Figure 3. Some of the most common disorders that are tested for by carrier testing are cystic fibrosis, Tay–Sachs disease, and hemophilia.

Tay–Sachs disease is a fatal genetic disorder that destroys nerve cells in the brain and spinal cord in young children. If a couple where both the mother and the father have a family history of Tay–Sachs disease are trying to conceive a child, their first step would be to request genetic screening to determine if they could pass on the disorder to their children.

Requesting carrier testing in the prepregnancy stage allows for the parents to determine how to proceed with the pregnancy. One option for parents that have a positive carrier test is the use of in vitro fertilization to avoid passing on a gene for a lethal genetic disorder. In vitro fertilization (IVF) is the process of fertilizing a human egg with a human sperm cell outside the body. Another option is that the embryos from the carrier of a genetic disorder are replaced with donor eggs or sperm in order to form the embryo. With IVF, the embryos that are created can be genetically screened to ensure they are free of genetic changes before being transferred to the uterus where pregnancy will proceed as normal.

Preimplantation testing is the type of genetic screening test used to determine if the embryo is free of any genetic changes that will lead to lethal disorders before being implanted into the uterus wall. In preimplantation testing, a small number of cells are taken from all the embryos and are tested for genetic changes that would indicate a genetic disorder. In Figure 4, we can see an up close example of how a single cell can be used to identify any chromosomal abnormalities. Then, only the embryos without these changes are implanted in the uterus to initiate a pregnancy.

The next stage of possible genetic screening is after conception and moves the focus from the expectant and preparing parents to the growing fetus. Genetic screenings are routinely performed during pregnancy as a part of prenatal checkups before the offspring is born. The word prenatal means “before birth.” The prefix pre- means “before” and the root of the word, natal, is Latin for “birth”. Prenatal is the term used to describe any point after conception and before birth.

Genetic screening performed in the prenatal stage helps detect changes in the genes or chromosomes of the offspring before birth. Prenatal testing is most recommended for families that are feeling uncertain about the genetic health of the offspring. Prenatal testing cannot identify all possible inherited disorders and birth defects, but it can help lessen the anxiety of expectant parents. Some common reasons that prenatal testing is offered include the following:

• Parents are high-risk for genetic disorders
• Previous pregnancies resulted in offspring with genetic disorders
• A family history of genetic disorders, like thalassemia or cystic fibrosis

The most common types of prenatal tests performed are amniocentesis and chorionic villus sampling, or CVS. Amniocentesis is a very accurate test for detecting changes in chromosomes, like an increase or decrease in the number of chromosomes. In amniocentesis, a long needle is inserted through the mother’s abdomen to collect a sample of amniotic fluid, which is the fluid that surrounds the developing fetus. The amniotic fluid is tested for chromosomal differences, such as an increase in the number of chromosomes seen in Down syndrome. Figure 5 shows an example of the process of amniocentesis testing. In contrast to amniocentesis is chorionic villus sampling (CVS). With CVS, a sample of tissue is tested for differences in the chromosomes.

Although amniocentesis and CVS are both performed in the prenatal testing, they are equally safe. The biggest difference between the two types of prenatal genetic screening tests is timing. It is also good to know that although these prenatal tests do a very good job of identifying a large number of the possible disorders, they cannot identify all of the possible inherited genetic disorders that may appear in the offspring. This is why genetic screening is also performed soon after birth, when the offspring is a newborn baby.

Newborn genetic screenings are used to test babies one or two days after birth to find out if they have certain diseases known to cause problems with health and development. By identifying genetic disorders early in life, treatment can be started as early as possible and the newborn can have better outcomes. Newborn genetic testing typically takes the form of a heel prick test, shown in Figure 6, to collect samples of blood to test for genetic disorders. A well-known example of a genetic disorder that is screened for in newborns is phenylketonuria (PKU). When a newborn has PKU, the brain fails to develop properly, leading to severe brain damage. However, if identified early, babies that test positive for PKU can be treated and will not have any brain damage.

After the newborn stage, most people do not have a need for a genetic screening. In some special cases, however, a person may need to know their risk for developing a disease that exists in their family. In these cases, presymptomatic testing is used. Presymptomatic testing (also called predictive testing) is used to detect gene changes associated with disorders that appear after birth, often later in life. The results of presymptomatic testing provide information about the risk of developing a specific disease before any signs or symptoms appear, which is also why this type of testing is called predictive.

Huntington’s disease is a genetic disorder that most commonly appears in a person around 35–40 years of age and leads to deterioration of cognitive function. Therefore, parents may produce offspring before realizing they carry the gene for Huntington’s disease. In this case, the child of someone who has just been diagnosed with the disease may want to know their chances of developing it. The type of testing this person would use in order to find out if they will also develop Huntington’s disease would be presymptomatic testing.

Genetic screening is an important type of testing that helps to determine the risk of developing certain diseases or inheriting genetic disorders. While extremely helpful, genetic screening is not foolproof—while it is a very powerful tool for information, it cannot provide results with certainty. Genetic testing can provide only limited information about an inherited condition. It cannot determine if a person will show symptoms of a disorder, how severe the symptoms will be, or whether the disorder will progress over time. As genetic tests can reveal information, such as a genetic disorder or disease for which there is no cure or even treatment, most of them are voluntary. This way, every individual is personally in control of knowing and unlocking the secrets of their own genetic makeup.

Let’s summarize what we have learned in this explainer.