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Key points
- Genetic testing can accurately predict a small number of traits and diseases.
- Genes reveal limited information about complex traits and behaviors.
- For many traits, a person’s environment has a strong influence on outcomes.
The popularity of genetic testing has surged in recent years and is predicted to be a $2.5 billion industry by 2024. Commercial companies have developed affordable, easy-to-use kits that allow customers to ship them a small sample of saliva. After extracting DNA from cellular material in the saliva, specific genes linked to physical traits or disease are analyzed.
With more than 26 million people using these services, it is clear that many are curious to see what secrets their DNA may hold. It also underscores the need to highlight the strengths and weaknesses of these genetic tests. What can they really tell us about people?
What Is a Gene, Exactly?
News agencies frequently announce, “Gene for X discovered,” in which “X” can be cancer, violence, intelligence, sexual orientation, or even spirituality. This is a very misleading way to talk about genes.
Genes are sections of our DNA that encode for proteins that carry out the functions of our body’s cells. Humans have approximately 22,000 genes scattered across 46 chromosomes. Since half our DNA comes from mom and the other half from dad, the majority of genes exist in pairs, or alleles. The alleles can be the same (homozygous) or they can differ (heterozygous). Heterozygous alleles mean that two different versions of the protein can be made in the cell.
Not only might alleles differ between parents, but genes can also incur mutations. DNA replicates itself prior to being packaged into sperm or egg, and sometimes mistakes happen. Such mutations, or variations, in the gene sequence can affect the function of the protein it makes in an adverse way that puts the individual at risk for disease.
The upshot is that genes make proteins. Whether the protein leads to a certain condition needs to factor in the function of that protein and how it operates in the context of the thousands of other proteins in the cell, which is one reason why predicting outcomes from genetic tests can get dicey.
What DNA Tests Can Tell You
Single genes have been linked to specific conditions, including some physical traits or diseases; however, this seems to be the exception to the rule. In these limited cases, the genetic test can be virtually 100 percent accurate.
Physical traits that arise from a single gene are called “monogenic,” but such traits are very limited in number, and ongoing research often uncovers additional genes that come into play. Nonetheless, well-characterized variations in some genes lead to highly predictable effects. For example, a gene called OCA2 makes a protein that produces the darkening pigment, melanin. Variations in this gene reduce melanin in the iris, resulting in blue eyes instead of brown.
Monogenic diseases are quite rare. A classic example is cystic fibrosis, which arises from mutations in a gene called CFTR. This gene makes a protein channel that helps regulate the amount of water that moves through cells and tissue. Insufficient water in tissues like the lungs or digestive tract leads to thick and immobile mucus. Mucus needs to be watered down so it can coat our innards, which allows proper function and protection from infectious germs.
Other examples of monogenic diseases include sickle cell anemia, Huntington’s disease, and Rett syndrome.
What DNA Tests Cannot Tell You
Most traits and diseases are “polygenic,” arising from more than one gene. As more genes are involved, predicting the effects of their combined influence gets increasingly difficult and less reliable. Complicating matters further is the crucial role that the environment plays in shaping complex traits.
Height is a physical trait that would seem to be reasonably well-predicted by genes. However, there is no single gene for height. A 2020 study identified nearly 10,000 gene variants associated with height. Furthermore, in contrast to a trait like eye color, an individual’s environment contributes to height. The quality of a child’s diet can be a strong driver of how tall a person becomes.
Researchers have found hundreds of genes linked to various types of cancer. Two of the most publicized are BRCA1 and BRCA2, which make proteins that suppress tumor formation by repairing damaged DNA. Mutations in these genes may increase the risk of developing breast cancer or other cancers, but not always. Similarly, having normal copies of BRCA1 and BRCA2 does not mean the person is immune from developing breast cancer—it can arise from faults in other genes. A family history of cancer is a better indicator of a person’s risk.
Much less is known about neurological diseases and psychological disorders, such as depression, schizophrenia, or autism. These conditions appear to have a genetic component, but it remains unclear which genes are involved, how they cause disease, and how much the environment comes into play.
Predicting personality and behavior is far more challenging, as these characteristics arise from interactions between massive gene networks and are strongly shaped by environmental factors. For example, variations in a gene called MAOA have been linked to antisocial behavior, but the association is strongest if the individual experienced abuse as a child. Intelligence also shows a high degree of malleability over genetics. As nutrition can affect height, access to high-quality education can affect intelligence.
These examples illustrate an important lesson about genes: They function in the context of other genes and their activity can be affected by the environment. You can think of your genes as a recipe, but the environment is the chef. Two different chefs can follow the same recipe, but their dishes will not necessarily taste the same.
The Bottom Line
It is vital to clarify the pros and cons of genetic testing because there is evidence that the results can lead to a self-filling prophecy. In one study, participants who were misled into thinking that they have genes for a certain behavioral trait began to act out the lie. After being misinformed that they had a genetic predisposition for lower exercise capacity, participants showed less endurance during a treadmill exercise.
Most of the genetic tests on the market are not going to reveal insights that the customer doesn’t already know or can’t deduce from family history. For those who do not have access to their biological family’s history, genetic tests could be more enlightening.
Barring a few exceptions, genes cannot be used as a crystal ball to tell much about a person’s future. This is not to say that genetic analyses are worthless. Even for complex, polygenic traits, predictions are expected to get better over time as scientists obtain more data and continue research into how genes interact with one another and their environment. Given the uncertainties involved, results of genetic testing are best interpreted with the help of a doctor or genetic counselor.
