Researchers often compare the differences between identical and fraternal twins to better understand health and behavior.
The first great insight is that genes and environments are almost always combined to affect our life path. Sometimes the biggest factor is genetics (think genetic disorders). Sometimes it’s environment (think infections). For the most part, it’s somewhere in between.
Such studies have accelerated the search for genes and environmental substances that cause or trigger diseases. This has helped us understand, treat and even prevent diseases. As twin research has matured, it has evolved to address important questions about when and how diseases occur.
So what has research from twins taught us about specific diseases and the human body?
Read more: Seeing twice: why twins are so important to health and medical research
1. Smoking increases the risk of bone fractures
Most studies linking the environment and disease are complicated by genetic factors. To get around this, we can work with twins who differ in environmental factors.
One such Australian study from 1994 compared 20 pairs of female twins, with only one of each pair being a long-term, heavy smoker.
The researchers found that smoking a pack of cigarettes a day for 20 years resulted in sufficient loss of bone density to cause osteoporosis. This doubled the risk of getting a bone fracture.
This provided convincing evidence that smoking causes osteoporosis and an increased risk of bone fractures.
2. Events around birth are not a major cause of epilepsy
Epilepsy is a group of disorders in which brain activity is abnormal and seizures are the prominent feature. Traditionally, the diagnosis was not possible until after a person’s first seizure, which can occur at any stage of life, from infants to the elderly.
Twin studies since the 1960s have shown that a mixture of genes and environment causes epilepsy. Until the early 1990s, however, it was believed that problems during childbirth were a major cause of epilepsy.
Obstetricians and midwives were often blamed for causing epilepsy. However, a twin study in 1993 did not support an association between minor problems during childbirth and the later development of epilepsy.
This information has helped physicians and their patients better understand the causes of epilepsy and not necessarily attribute the blame to the birth process.
Identical twins are different under the skin from before birth
Genetically identical twins almost always look identical. Yet, already at birth, they have accumulated differences in the structure and function of their genes.
These differences are caused by a mixture of random events and individual experiences in the womb.
The location a fertilized egg implants in the uterus is random, but in some places is more favorable for growth. For the subgroup of identical twins that divide before reaching the uterus, different locations can create different environments in which a baby develops.
As a result of this or other random events, about every sixth twin differs by more than 20% in weight at birth, which may be associated with an increased risk of disease at birth, especially for the smaller twin.
Such individual experiences could also help explain Brazilian twin couples where only one child was born with Zika virus infection.
4. Leukemia occurs before birth
Changes in the genetic sequence of blood cells can predispose people to develop leukemia (cancer of the blood).
Such changes are unique to each person, however when these changes happened to people used to be a mystery. That was until identical twin children were discovered with leukemias originating from the same cell.
Lymphocytes (white blood cells) in the immune system randomly mix their immune genes, making each person genetically unique, even identical twins.
Read more: Same same but different: when identical twins are non-identical
The researchers concluded that the leukemia started in one twin in the uterus and spread to the other twin through blood vessels in a common placenta.
However, while the first step against leukemia occurred before birth, the cancer progression was different among the twins, resulting in leukemia being diagnosed at different ages.
This provided the first evidence that some leukemias may lie dormant for years and enabled future research that could identify the events along this process.
5. Many twins do not know if they are identical or fraternal
Identical twins start as one fertilized egg that divides after a few days. They share almost 100% of their DNA and are almost always of the same sex.
Fraternal twins are due to two eggs fertilized around the same time. They are as genetically diverse as any sibling pair and may have the same or different sex.
In 2012, my colleagues and I at Twins Research Australia conducted a study at a national twin festival on couples who had some uncertainty about their genetic identity. We used “genetic fingerprints” on DNA from cheek inoculations provided by same-sex twins of all ages. This test is the definitive way to find out if twins are identical or fraternal.
We compared this to the perception of the twins themselves before taking the test.
We found that almost a third of the twins we tested had either been incorrect or uncertain about their genetic identity. Some had even been misinformed by doctors.
The universal feeling was twins, and their families felt better knowing the truth. Our data enabled us to develop better educational resources for twins and their advocates to know more about themselves.
Read more: Not all twins are identical, and it’s been an evolutionary puzzle until now