9.3.1: Genetic Disorders and the Ethics of Prenatal Testing
Genetic errors are relatively common in the process of conception, and are thought to be the cause of about 50% of known miscarriages (ACOG Committee on Practice Bulletins—Gynecology, 2021). Even if a fertilized egg implants successfully in the uterine wall and continues to grow, genetic mutations can cause a variety of problems. Since each embryo inherits half of their 46 chromosomes (DNA) from each parent, certain diseases can be inherited from only one parent if the genetic disorder is dominant - the parent will pass that trait on to approximately half of their offspring. Some examples include Huntington’s disease (a brain disease), and certain types of dwarfism (achondroplasia) and gigantism (Marfan syndrome). Recessive genes are associated with some diseases that will affect a child if both parents carry them, and if both of those recessive genes are passed on (Schneider, 2020). See Fig. \(\PageIndex{1}\) below for the “Punnett Square” diagram illustrating recessive genes.
Examples of diseases associated with recessive genes include:
- Tay-Sachs disease: a neurological disorder, most common in people with Ashkenazi Jewish ancestry;
- Sickle-cell anemia: causes red blood cells to be shaped like a crescent and decreases oxygen transportation, most common in people with African or African American ancestry;
- Cystic fibrosis: a disease of the lungs causing excessive mucus production and damage to lung tissue, most common in people with northern European ancestry;
- Thalassemia: which causes a lack of hemoglobin in the blood, decreasing oxygen transport capacity, affects those with Mediterranean, Middle Eastern, Asian, and African ancestry, (CDC, 2020, National Health Service, 2021, Schneider, 2020).
Still other genetic disorders are linked to the X chromosome or female sex chromosome. These diseases primarily affect genetic males who have XY chromosomes, since the affected X tends to be recessive (whereas in a genetic female with two X chromosomes, the recessive gene would not be expressed) (Schneider, 2020). For example, Duchenne muscular dystrophy is an X-linked disorder that leads to muscular degeneration, weakness, and eventually paralysis. It is not more common in any racial or ethnic populations, but much more common in genetic males than females (Muscular Dystrophy Association, 2017).
Some genetic disorders are not necessarily inherited but occur due to a genetic mishap in either the sperm, egg, or both prior to conception. The human gametes (egg and sperm) each contain one half of 23 pairs of chromosomes, and the resulting zygote (fertilized egg) will have 46 chromosomes, or 23 pairs. A trisomy occurs when there are 3 copies of one of the chromosomes, perhaps from the egg or the sperm carrying both chromosomes in a pair, instead of one. The most common expressions are trisomy 13 (Patau syndrome), trisomy 18 (Edwards syndrome) and trisomy 21 (Down syndrome). Infants born with trisomy 13 or 18 rarely survive past one year old, due to resulting malformations of the brain, heart, lungs, and kidneys (Stanford Medicine Children’s Health, n.d.). Down syndrome causes a spectrum of physical, mental, and developmental challenges, and can increase the risk of congenital heart defects, hearing loss, and sleep apnea. However, most people with Down syndrome can live long and healthy lives with the appropriate support (CDC, 2024d). A monosomy occurs when there is a chromosome missing from a pair. Turner syndrome is caused by a partial or complete deletion of the second X chromosome in a genetic female. This syndrome causes suppressed physical growth, and possibly future reproductive and other health issues (Boston Children’s Hospital, n.d.).
Prenatal testing can identify potential genetic disorders in the first or second trimester. Prenatal screening is done during the first trimester, and involves a blood test that checks fetal DNA matter found in the mother’s bloodstream for genetic mutations like trisomies or monosomies. Nuchal translucency screening (NTS) is an ultrasound that measures the thickness of the neck of the fetus, as more fluid buildup can indicate an increased risk of trisomies. An anatomical ultrasound scan is also typically done around week 20 of pregnancy to measure fetal growth and screen for malformations of the head, internal organs, or limbs (ACOG, 2023b). Diagnostic tests cannot typically be done until the second trimester, and include taking samples from the amniotic fluid (amniocentesis) or placenta (chorionic villus sampling). While screening tests are highly accurate at predicting the risk for specific genetic disorders or malformations, diagnostic tests are usually used to confirm the presence of a disorder. Taking into account other risk factors like the parents’ family history or genetic risks, and maternal age (pregnancy at over age 35 tends to increase certain risks), physicians may recommend less-invasive prenatal screening first, and then diagnostic testing if the screens indicate an increased risk (Johns Hopkins Medicine, 2021).
Genetic testing of parents prior to pregnancy and prenatal screening or diagnostics during pregnancy can help inform parents’ decision making and family planning. Some parents may decide to not have children or adopt children if they are genetic carriers for certain syndromes. Considering a diagnosis where an infant would likely suffer and have a short life expectancy after birth, some parents may opt to terminate a pregnancy. Sometimes when a diagnosis is discovered and the pregnancy is continued, parents can make preparations for services and support for both themselves and an infant with a disability. Still other parents may prefer to not know a diagnosis until birth, as it would not change their plans. Regardless of parents’ healthcare decisions, it is important that parents have access to accurate information regarding screening, diagnostic tests, and the potential impacts (ACOG, 2023b, Schneider, 2020).
Genetic screening also comes with ethical questions from a population perspective. Although substantial suffering and death from cruel diseases could potentially be prevented with genetic screening, emerging gene therapies, early medical intervention, and/or family planning, there is also the real risk of racism and ableism affecting genetic medicine. Historically, eugenics and racialized medicine have been used to justify atrocities: the Holocaust in Nazi Germany, forced sterilization of Black women and immigrants in the U.S., and other attempts at ethnic cleansing and harming or killing those with disabilities around the world. Even when genetic screening and counseling is done ethically, there is a risk of genetic information influencing a person’s social life, their potential for employment, and thus also their ability to get health insurance coverage. Genetic testing and prenatal screening therefore should always be voluntary and confidential, include education and counseling, and include tests that are of high quality and evidence-based (Schneider, 2020).