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Identical Twins: Not So Identical After All

Do identical twins have the same DNA?

With Rita Cantor PhD

Two young female twins share dessert

How much of what makes you who you are is determined by your genes or your environment? Why might one person get cancer or heart disease and another not — is it their genetic profile, what they ate (or didn’t eat), how much (or how little) they exercised or other lifestyle factors? In other words, it’s the age-old nature-nurture debate, with nature referring to our genes that we inherited from our parents and nurture meaning all the variables in our environment, from whether we worked night shifts, spent a lot of time suntanning or rarely exercised. Hoping to untangle the relationship between nature and nurture is why scientists have been studying identical twins for decades.

What is an identical twin?

While twins that don’t look alike are referred to as fraternal twins, identical twins typically look, well, identical. The scientific term for identical twins is monozygotic twins. To understand why they are given that name, it’s important to understand how they form. First, one sperm unites with an egg. The egg contains the genetic material (DNA with genes, which are individual DNA pieces carrying the inherited information about you) from your mother with the genetic material from your father (found in the sperm). The result is a fertilized egg (now a single cell called a zygote) that repeatedly divides, forming a clump of cells that eventually splits to form two embryos. Because they came from one zygote, these twins are called mono (the prefix for “one”) zygotic twins, aka identical twins.

Compare this process with the development of fraternal twins, known by the scientific name dizygotic twins — with the prefix “di” meaning two. They form when two eggs are fertilized by two sperm, resulting in two zygotes. Each repeatedly divides into two clumps of cells that form two embryos.

Semi identical twins and other types of twins

Despite the name, identical twins are rarely completely identical. Their skin tone, weight, height or personality, to name a few characteristics, may be different. How does this happen?

Every time a cell divides — as occurs in the developing embryo — your DNA must replicate, a very complicated process. According to Rita Cantor PhD, Professor Emeritus of human genetics at the David Geffen School of Medicine at UCLA, “Cell replication is not a perfect process, and mistakes occur.” An error — and one not corrected by the body’s repair mechanisms — results in a tiny change in the DNA, referred to as a mutation. A mutation can occur from the very first time the cell divides after the egg is fertilized, or it may happen later, after the two “identical” embryos form.

Dr. Cantor explains that in most instances, a pair of identical twins share the same DNA when they split. However, she continues, a recent report found that some developing twin embryos may already have genetic differences. This study conducted in Iceland looked at almost 400 sets of identical twins, finding that 12% of the identical twins they examined had up to 100 genetic differences. But this is still thought to be a miniscule difference, considering that an individual’s genetic code is huge. In effect, though identical twins don’t have identical DNA, they are genetically similar.

How are twins made identical or fraternal?

There can be differences between the twins because of the interactions in how the environment interacts with their genetic material. In other words, the cells in one of the twins may express their DNA one way compared with how it’s expressed in the other twin.

Even though DNA can be seen as a recipe— actually a book of recipes— for everything about you, many factors can affect how the recipes are executed or expressed. It’s like having several different pastry chefs baking a chocolate fudge cake. They may rely on the same recipe, but the cakes end up a wee bit different because of different chef techniques, different ovens and so forth.

Even if the twins had identical DNA, the environment (even in the womb) influences how their genes are expressed. This can lead to differences in personality, health and appearance. Scientists have found that a variety of environmental factors (such as smoking, diet, exercise and stress levels) can affect our genes by boosting the activity of a gene, slowing it down or even curtailing its activity altogether.

According to Alexandria Haseley MS, a Genetic Counselor at the Cleveland Clinic’s Genomic Medicine Institute, “A lot of research has demonstrated that genetic differences become greater as we age.” That’s because of an accumulation of errors in DNA as well as the effects of environmental factors. 

Do most identical twins have the same risk for different diseases?

Not necessarily. Identical twins may differ on a host of diseases and conditions. According to Haseley, “If a genetic change occurs in a gene that affects its function, health concerns may arise.” Twins may differ in their risk of cancer, rheumatoid arthritis or other diseases. When scientists studied schizophrenia in identical twins, they found that there were differences between the twins in terms of genes associated with the condition, with one of the two having the disease and the other not. If, for example, a genetic change occurs in a gene associated with heart defects, that individual may develop a heart condition, or if one twin has a gene affecting cancer risk, they may have an increased chance for developing that cancer, but other factors like the environment also play a role.

Dr. Cantor explains that one of the twins may have a particular disease if a change occurs in a gene predisposing to that disease and the twin’s environment also contributes to an increased risk of that condition. For example, one twin might have high triglyceride levels (blood fats) if they have genetic changes in the liver, which is involved with triglyceride metabolism, and if that twin also eats a high-fat diet. On the other hand, if both of the twins had identical DNA but one of the twins gains a lot of weight, that factor can also trigger a rise in triglycerides.

Can genetic differences affect how one twin reacts to COVID-19?

Yes. According to Dr. Cantor, “Most recently, twin differences have been observed for the response to infection with the SARS-CoV-2 virus, the virus responsible for COVID-19.” For example, doctors have seen that one of the identical twins may become so ill that they need to be hospitalized while the other has a relatively mild case of the disease. This could be because of differences in how the individual twin’s immune system operates to defend against the invading virus or how the body’s defenses warn other cells about the presence of the virus. Maybe it’s because a genetic difference makes it easier for the virus to enter the cells or for the immune system to overreact to the invading viruses — something that can cause extensive tissue damage — or whether the body produces a high fever and body aches in response to the infection.

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