Amniocentesis is a prenatal diagnostic procedure in which a small sample of the amniotic fluid that surrounds the fetus is extracted from the mother’s uterus through a long, hollow needle that is guided by ultrasound as it is inserted into the mother’s abdomen (Odibo, 2015). The amniotic fluid contains fetal cells, which are grown in a laboratory dish to create enough cells for genetic analysis. Genetic analysis is then performed to detect genetic and chromosomal anomalies and defects. Amniocentesis is less common than ultrasound, as it poses greater risk to the fetus. It is recommended for women aged 35 and over, especially if the woman and partner are both known carriers of genetic diseases (Vink & Quinn, 2018a). Usually amniocentesis is conducted between the 15th and 18th weeks of pregnancy. Conducted any earlier, an amniocentesis may increase the risk of miscarriage (Akolekar et al., 2015). Test results generally are available about 2 weeks after the procedure because it takes that long for the genetic material to grow and reproduce to the point where it can be analyzed.
Chorionic villus sampling (CVS) also samples genetic material and can be conducted earlier than amniocentesis, between 9 and 12 weeks of pregnancy (Vink & Quinn, 2018b). CVS requires studying a small amount of tissue from the chorion, part of the membrane surrounding the fetus. The tissue sample is obtained through a long needle inserted either abdominally or vaginally, depending on the location of the fetus. Results are typically available about 1 week following the procedure. CVS is relatively painless and, like amniocentesis, has a 100% diagnostic success rate. Generally, CVS poses few risks to the fetus (Shim et al., 2014). However, CVS should not be conducted prior to 10 weeks’ gestation as some studies suggest an increased risk of limb defects and miscarriages (Shahbazian, Barati, Arian, & Saadati, 2012).
During amniocentesis, ultrasound is used to guide the insertion of a long, hollow needle into the mother’s abdomen in order to extract a sample of the amniotic fluid that surrounds the fetus. The amniotic fluid contains fetal cells, which are grown in a laboratory dish and tested for genetic and chromosomal anomalies and defects.
Saturn Stills/Science Source
Noninvasive prenatal testing (NIPT) screens the mother’s blood to detect chromosomal abnormalities. Cell-free fetal DNA (chromosome fragments that result in the breakdown of fetal cells) circulates in maternal blood in small concentrations that can be detected and studied by sampling the mother’s blood (Warsof, Larion, & Abuhamad, 2015). Testing can be done after 10 weeks, typically between 10 and 22 weeks. Given that the test involves drawing blood from the mother, there is no risk to the fetus; however, NIPT cannot detect as many chromosomal abnormalities as amniocentesis or CVS and with less accuracy (Chan, Kwok, Choy, Leung, & Wang, 2013; National Coalition for Health Professional Education in Genetics, 2012). Researchers have identified the entire genome sequence using NIPT, suggesting that someday, NIPT may be as effective as other, more invasive techniques (Tabor et al., 2012). Pregnant women and their partners, in consultation with their obstetrician, should carefully weigh the risks and benefits of any procedure designed to monitor prenatal development.
Prenatal Treatment of Genetic Disorders
What happens when a genetic or chromosomal abnormality is found? Advances in genetics and in medicine have led to therapies that can be administered prenatally to reduce the effects of many genetic abnormalities. Fetoscopy is a technique that uses a small camera, inserted through a small incision on the mother’s abdomen or cervix and placed into the amniotic sac that encases the fetus, to examine and perform procedures on the fetus during pregnancy. Risks of fetoscopy include infection, rupture of the amniotic sac, premature labor, and fetal death. However, when serious abnormalities are suspected, fetoscopy permits a visual assessment of the fetus, which aids in diagnosis and treatment. Hormones and other drugs, as well as blood transfusions, can be given to the fetus by inserting a needle into the uterus (Fox & Saade, 2012; Lindenburg, van Kamp, & Oepkes, 2014). Surgeons rely on the images provided by fetoscopy to surgically repair defects of the heart, lung, urinary tract, and other areas (Deprest et al., 2010; Sala et al., 2014).
In addition, researchers believe that one day, we may be able to treat many heritable disorders thorough genetic engineering, by synthesizing normal genes to replace defective ones. It may someday be possible to sample cells from an embryo, detect harmful genes and replace them with healthy ones, and then return the healthy cells to the embryo where they reproduce and correct the genetic defect (Coutelle & Waddington, 2012). This approach has been used to correct certain heritable disorders in animals and holds promise for treating humans.
Thinking in Context 2.3
Suppose you are a 36-year-old woman pregnant with your first child. Considering the types of prenatal diagnostic testing described in this section, what would be the advantages and disadvantages of each? What factors would influence a health care provider’s recommendations for prenatal testing?
Heredity and Environment
We have learned a great deal about genetic inheritance. Most human traits, however, are influenced by a combination of genes working in concert with environmental influences. Our genes, inherited from our biological parents, consist of a complex blend of hereditary characteristics known as genotype. Our genotype, or genetic makeup, is a biological influence on all of our traits, from hair and eye color to personality, health, and behavior. However, our phenotype, the traits we ultimately show, such as our specific eye or hair color, is not determined by genotypes alone. Phenotypes are influenced by the interaction of genotypes and our experiences.
Behavioral Genetics
Behavioral genetics is the field of study that examines how genes and experience combine to influence the diversity of human traits, abilities, and behaviors (Krüger, Korsten, & Hoffman, 2017; Plomin et al., 2013). Genotypes alone do not determine people’s traits, characteristics, or personalities; instead, development is the process by which our genetic inheritance (genotype) is expressed in observable characteristics and behaviors (phenotype). Behavioral geneticists recognize that even traits that have a strong genetic component, such as height, are modified by environmental influences (Dubois et al., 2012; Plomin, DeFries, Knopik, & Neiderhiser, 2016). Moreover, most human traits, such as intelligence, are influenced by multiple genes, and there are often multiple variants of each gene (Bouchard, 2014; Chabris, Lee, Cesarini, Benjamin, & Laibson, 2015; Knopik et al., 2017).
Methods of Behavioral Genetics
Behavioral geneticists devise ways of estimating the heritability of specific traits and behaviors. Heritability refers to the extent to which variation among people on a given characteristic is due to genetic differences. The remaining variation not due to genetic differences is instead a result of the environment and experiences. Heritability research therefore examines the contributions of the genotype but also provides information on the role of experience in determining phenotypes (Plomin et al., 2016). Behavioral geneticists assess the hereditary contributions to behavior by conducting selective breeding and family studies.
Using selective breeding studies, behavioral geneticists deliberately modify the genetic makeup of animals to examine the influence of heredity on attributes and behavior. For example, mice can be bred to very physically active or sedentary by mating highly active mice only with other highly active mice and, similarly, breeding mice with very low levels of activity with each other. Over subsequent generations, mice bred for high levels of activity become many times more active than those bred for low levels of activity (Knopik, Neiderhiser, DeFries, & Plomin, 2017). Selective breeding in rats, mice, and other animals such as chickens has revealed genetic contributions to many traits and characteristics, such as