Are Twins Genetically Identical?

One of the most common questions asked online is: "Are twins genetically identical?" Most people would assume twins are genetically identical, but research suggests that may not always be true.

How Are Identical Twins Formed?

Twinning refers to the formation of twins during conception. There are two types of twins: fraternal twins and identical twins.

Fraternal twins form when one sperm cell fertilizes an egg and, at the same time, another sperm cell fertilizes a different egg cell. This creates two different fertilized eggs during the same pregnancy.

Identical or monozygotic twins occur when one egg is fertilized by one sperm, and this single fertilized egg then splits into two. Since identical twins arise from the same fertilized egg, it makes sense that they would have identical DNA.

Over the years, twin studies have tried to explain why many twins end up with differences. Until recently, environmental factors were considered the primary answer to those differences between genetically identical people.

Genetic Differences in Identical Twins

Researcher Michael Bruder found differences in copy number variation (CNV) between identical twins. CNV refers to sections of the genome that are repeated, and the number of repeats can vary from person to person. While studying twins, Bruder and his colleagues found CNV differences between identical twins.

In one study, Bruder compared genetic differences in two sets of identical twins where one sibling in each pair suffered from a condition. One set had one twin with an increased risk of leukemia. That twin did in fact develop leukemia while the other did not.

From his study, Bruder believed that studying the genome of twins could shed more light on human genetics and diseases. When one twin suffers from a particular condition and the other does not, analyzing genetic differences may provide insight into which part of the genome points to an increased risk for that condition.

Charles Lee, a geneticist at Brigham and Women's Hospital in Boston, argues that genetic differences are not necessarily present at birth. Environmental factors such as radiation or other carcinogens can break double-strand DNA, leading to genetic variations. As people age, more genetic differences emerge, including between twins. Lee believes that is what makes twins different, not differences that developed in the womb.

Can Identical Twins Be Genetically Different?

Research published in Nature Genetics found that genetic differences are responsible for many of the differences seen in pairs of identical twins. These differences, known as somatic mutations, are not inherently good or bad, but they can influence many aspects of a person including appearance and susceptibility to diseases. Gene mutations typically happen during cell division when an error occurs as DNA replicates.

In one of the twin pairs studied, a mutation was found in all of the cells of one sibling's body. This happened during the early stages of development for that twin but not the other. This significantly affects the twin's genetic code, which is why one twin may look different from the other, and why one could develop a genetic condition while the other does not.

The research will likely lead to other twin studies that help explain how twinning happens. It may also begin to change how we define "identical" twins, since in many cases they may only be semi-identical.

Nature vs. Nurture

For many years, researchers have attributed differences between identical twins primarily to environmental factors. Only a select few scientists have argued that identical twins can be genuinely genetically different, and have been able to demonstrate this in their study results.

The nature vs. nurture debate in twin research remains active. The only solution is to continue studying the DNA of identical twins to identify differences in the genome and monitor what happens to them over their lifetimes.

These longitudinal studies may take many years to produce results, but those results will be used for decades to advance understanding of genetics, disease susceptibility, and human development.

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