What are some general findings from studies on identical and fraternal twins?

Chromosome 3 pairs in each set of twins are digitally superimposed. One twin's epigenetic tags are dyed red and the other twin's tags are dyed green. When red and green overlap, that region shows up as yellow. The 50-year old twins have more epigenetic tags in different places than do 3-year-old twins.

The insight we gain from studying twins helps us to better understand how nature and nurture work together. For well over a century, researchers have compared characteristics in twins in an effort to determine the extent to which certain traits are inherited, like eye color, and which traits are learned from the environment, such as language. Typically taking place in the field of Behavioral Genetics, classical twin studies have identified a number of behavioral traits and diseases that are likely to have a genetic component, and others that are more strongly influenced by the environment.

Depending on the study and the particular trait of interest, data is collected and compared from identical or fraternal twins who have been raised together or apart. Finding similarities and differences between these sets of twins is the start to determining the degree to which nature and environment play a role in the trait of interest.

Twin studies have identified some traits that have a strong genetic component, including reading disabilities like dyslexia. Other traits, like arthritis, are more likely influenced by the environment.

Twin studies uncover genetic and environmental contributions

Twins share the same genes but their environments become more different as they age. This unique aspect of twins makes them an excellent model for understanding how genes and the environment contribute to certain traits, especially complex behaviors and diseases.

For example, when just one twin gets a disease, researchers can look for elements in the twins' environments that are different. Or when both twins get a disease, researchers can look for genetic elements shared among similar twin pairs. These types of data are especially powerful when collected from large numbers of twins. Such studies can help pinpoint the molecular mechanism of a disease and determine the extent of environmental influence, potentially leading to the prevention and treatment of complex diseases.

To illustrate, for twins with schizophrenia, 50% identical twins share the disease, while only about 10-15% of fraternal twins do. This difference is evidence for a strong genetic component in susceptibility to schizophrenia. However, the fact that both identical twins in a pair don't develop the disease 100% of the time indicates that other factors are involved.

Identical twins (left) share all their genes and their home environment. Fraternal twins (right) also share their home environment, but only half of their genes. So a greater similarity between identical twins for a particular trait compared to fraternal twins provides evidence that genetic factors play a role.

Comparing Identical and Fraternal Twins: A higher percentage of disease incidence in both identical twins is the first indication of a genetic component. Percentages lower than 100% in identical twins indicates that DNA alone does not determine susceptibility to disease.

References

References

Wong A.H., Gottesman I., Petronis A. (2005) Phenotypic differences in genetically identical organisms: the epigenetic perspective. Human Molecular Genetics, 14: Review Issue 1, R11-R18.

Fraga, M.F. et al. (2005) Epigenetic differences arise during the lifetime of monozygotic twins. PNAS, 102:10604-9.

Poulsen P., Esteller M., Vaag A., Fraga M.F. (2007) The Epigenetic Basis of Twin Discordance in Age-Related Diseases. Pediatric Research, 61: 38R-42R (subscription required).

Adrián graduated with honors with a Bachelor’s in Genomic Sciences in 2016 from the National Autonomous University of Mexico. He performed a research internship at the Swiss Federal Polytechnical Institute studying antibiotic combination therapies. Currently, he is working toward a Ph.D. in genetics at the QIMR Berghofer Medical Research Institute, where he is working on finding the genetic and environmental risk factors for certain mental disorders and treatment response.

Brittany L. Mitchell

Brittany completed her M.Sc. in Genetics in 2016 at the University of Pretoria, South Africa. She is now in the first year of her Ph.D. in at QIMR Berghofer Medical Research Institute, where she is examining the genetic and environmental risk factors behind dementia and cognitive impairment. She is currently working on a large twin study that aims to understand the genetic and environmental correlations between vitamin D, skin color, and sun exposure.

Miguel E. Rentería

Miguel is a human geneticist at the QIMR Berghofer Medical Research Institute. He is interested in understanding how genes explain individual differences in brain structure and predisposition to brain diseases. Previously, he obtained his Ph.D. from the University of Queensland and conducted postdoctoral training in both Australia and the United States. *[email protected]

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Abstract

Have you ever wondered why you look different from other people? Differences, such as height and hair color can come from differences in your genes called mutations. Understanding how mutations shape us can be a bit tricky, because we are also affected by the world around us. Twins and siblings can help us understand whether people are different because of genetic differences or because they live in different environments. Identical twins share the same genes, while non-identical twins share, on average, half of their genes with the other twin. We can measure and compare the degree of similarity in a trait between pairs of identical and non-identical twins, to understand how much of the difference in a trait between people is caused by genes and how much is caused by the environment.

Every person in the world is different from every other. You may have different colored hair, skin, or eyes, or a different height, along with a whole bunch of other traits that make you look unique. On the other hand, you share very similar traits with members of your family. You might have the same hair color as your dad and the same eye color as your mom. You might look a bit like your brother or sister. This is because families share some variations (also called mutations) in their genes.

We could say that genes are small pieces of biological information, passed from parents to their children, encoded in the DNA. Think of genes as a recipe with instructions that list ingredients, amounts, and steps for how to make food. In this example, you are the food, and your DNA is both the paper and the letters used to write the recipe. Although our genes are like recipes, keep in mind that it is a very difficult recipe, with many (many!) ingredients and steps that we are still trying to discover and understand. Now, there are many types of food in the world, just like there are many types of people. Cupcakes are different from cookies because their recipes are different. People are different from each other because of differences in their genes (recipes). Some people are more similar than others (for instance, you and your brother or sister look more alike than you and your friends). This would be like cupcakes and muffins—they are not the same, but they are pretty similar. This is because their recipes are alike but not exactly the same. Sometimes certain mutations in your genes can be bad for you and make you sick, while other mutations can protect you from disease. Some mutations may even be good or bad depending on the environment! In our analogy, a mutation that causes a disease would be like using the wrong recipe. If you put salt in your cake mix instead of sugar, your cake will not come out well. While you may think that a mutation is something bad (or maybe something cool that some superheroes have), the truth is that most mutations are neither good nor bad—they are called neutral.

A recipe is not the only thing that affects how a cake comes out. The environment also plays an important role. If you make the oven too hot the cake will burn, but if it is not hot enough, it will not cook and will flop. But wait! Using the wrong ingredients, or the correct ones in the wrong amounts, can also make a cake flop! [1]. If you find a flopped cake, you will not always know what caused the flopping. Was it a mistake in the recipe? Or was it cooked the wrong way? This is an important difference between genes and recipes. Although you can see the symptoms of a disease just like you can see a flopped cake, the recipe in the DNA is written in a different language and we cannot easily go back and read it to find the mistake.

The environment has a big effect on people’s traits. Think about skin color, for example. You might be born with the same skin color as your friend. If your friend enjoys playing sports outside and you like reading indoors, the sun will make your friend’s skin darker while yours stays untanned. You will look different, even though your skin color genes are the same. Then again, genes can also make people have different skin colors. When you see people with different skin colors, it is hard to tell whether those differences are caused by genes (genetic), caused by sun exposure (environmental), or even caused by a combination of both genes and the environment! Actually, all of the differences between you and all the people around you come from differences either in your genes or your environment, or a combination. For example, you might be very smart because your parents make you read a lot (this is an effect of your environment). Or you might be very smart because you have genes that affect your brain, allowing you to read more quickly or understand things more easily [2].

Understanding how mutations affect a specific trait in a group of people is very important, because it helps us find out why we are all different. Also, understanding mutations can help us study the way that genes cause some diseases. On the other hand, finding out how the environment affects a trait is also useful, because the environment can sometimes be changed. For example, scientists found out that the disease Diabetes is only influenced a little by genes (it has a heritability of 26%; [3]). Knowing this, doctors can help people to avoid getting the disease through environmental changes (like eating a healthy diet or exercising more), even if they have the genes for Diabetes!

To understand the effects of genes vs. the environment on a particular trait, we could examine a group of people and compare the differences in their genes and the differences in the trait of interest. This would be like comparing all the recipes used for different cakes to see if any specific ingredient is common to all the floppy cakes. If there is a common ingredient in floppy cakes, and that ingredient is not present in non-floppy cakes, then that specific ingredient is probably the cause of the cake flopping. In the same way, we can see if people with differences in a trait also all have specific mutations. We can then use maths to understand how much of the differences in a trait are explained by the mutations. The problem with this method is that reading the DNA of lots of people can be very expensive and complicated. Luckily, almost a 100 years ago, scientists found a powerful way to study the effect of genes vs. the environment without needing to read the DNA. This method uses twins [4].

Twins are a special type of siblings because they are born at the same time. There are two types of twins. Identical twins look very similar and are always either both boys or both girls. Non-identical twins may look different, and may even be a boy and a girl. Identical twins share all of their genes, while non-identical twins, just like non-twin siblings, share half of their genes. So, we can assume that any differences in traits between identical twins come from the environment, and not from differences in their genes. By measuring a trait (such as skin color) in large groups of twin pairs and siblings, we can understand the effect of genes vs. the environment on a trait without actually looking directly at people’s genes to see their mutations.

An example of results from two twin studies is shown in Figure 1. In one study, Australian twins (both identical and non-identical) had intelligence measured using an IQ (intellectual quotient) test. In the other study, twins were asked how many hours per night they typically sleep. Both studies aimed to find out how much genes contribute to the traits of interest (IQ and sleep time). To get an idea of how genetic a trait is, they compared how similar the pairs of identical twins were and how similar the pairs of non-identical twins were. Because identical twins share all of their genes, their measurements of IQ and sleep time will be more similar the bigger role genes play in it (i.e., differences between two identical twins must be caused by the environment because their genes are the same). On the other hand, we expect non-identical twins to be less similar than identical twins, but we still expect them to be somewhat similar because some of their genes are shared. Keep in mind that what is being compared is not the trait measurements, but rather how similar the twins are (both with high IQ, or both sleeping the same amount of time). In Figure 1, similarity can be observed by how close the dots (twin pairs) are to the green line in the middle (the perfect similarity line). These studies showed that IQ is very heritable (although the environment still plays a role in determining your IQ). We can tell this because the IQ of identical twins are almost always the same and non-identical twins are only sometimes the same. On the other hand, the graph for sleep of identical twins shows a big spread from the middle green line. This means that identical twins show big differences between them, and because they have the same genes this big differences are caused by the environment. This means that sleep length has a low heritability.

• Figure 1 - Twin data help us understand genetic and environmental effects.
• Results of two real-world twin studies. The two on the top are the results for IQ (a measure of intelligence), while the ones on the bottom show the results of sleep time. In the graphs, each dot is a pair of twins. The X axis represents the score (or sleep hours) the first twin got, and the Y axis represents the score the other twin (brother) got. If both twins are very smart, the dot representing them will be in the upper-right corner (they both got a high score). When a pair of twins share the exact same measurements (IQ or sleep hours) they will lay on the green line. If their measurements are very different, their dot will be far from the green line.

Studies like this have identified that personality, intelligence, poor eyesight, and even mental diseases, such as bipolar disorder and schizophrenia have a medium to high heritability (i.e., they are strongly influenced by genes). There is almost no limit to what kind of trait or disease can be studied. If it can be measured or classified, we can estimate its heritability! We study twins to understand how much of the difference in a trait between people is caused by genes and how much is caused by the environment. These studies are important because they help scientists quantify genetic and modifiable environmental factors that increase the risk of certain diseases. Scientists have done many studies like these. Around 18,000 human traits, including height, body weight, and several diseases, have been studied so far [5]. So, when you see a pair of twins, remember how genetically special and valuable they are for science and health research.

Glossary

Trait: ↑ A specific characteristic or feature of a person. For example, someone’s hair color or height.

Mutation: ↑ A form or version of a gene. Mutations can arise suddenly at any time and are usually neither good nor bad for you.

Gene: ↑ A piece of DNA that is passed from parents to children. Genes contain information about your traits.

DNA: ↑ Molecule inside your cells that stores all of your genes. Your DNA contains one copy of each of your parent’s DNA, making you similar to both of them.

Diabetes: ↑ Is a condition that occurs when the body cannot use glucose (a type of sugar) normally.

Heritability: ↑ The amount of change in a trait that is due to genetic differences.

Similarity: ↑ A score that measures how alike things are. In this case, we measure how alike each twin is with his co-twin.

Modifiable: ↑ Something that can be changed. For example, the amount of exercise you do.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Article information

Citation

Campos A, Mitchell B and Rentería M (2019) Twins Can Help Us Understand How Genes and the Environment Shape Us. Front. Young Minds. 7:59. doi: 10.3389/frym.2019.00059

Editor

Suzanne Phelan

Science Mentors

Cora Burt , Siew Kit Ng , Christopher Sylvester

Publishing dates

Submitted: August 27, 2018; Accepted: April 5, 2019; Published online: April 24, 2019.

Copyright © 2019 Campos, Mitchell and Rentería

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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What were the results of the twins study?

The Twins Study demonstrated the resilience and robustness of how a human body can adapt to a multitude of changes induced by the spaceflight environment. Researchers found that many of Scott's responses observed returned to preflight levels by the study's end.

What has research on identical twins discovered?

Identical twins are living proof of how genetics shapes our looks and traits. Now, researchers have found they carry a molecular signature on their DNA that no one else has—one that becomes fixed in their cells early in development and stays with them into adulthood.

What is the general result of research on the personalities of identical twins who are reared apart?

The research team found that identical twins who are reared apart had the same chance of being similar as twins who were raised together. Bouchard and his colleagues concluded that genetic factors have a large influence on behavioral habits demonstrating the influence of the genetics on development.

What are some things that could be determined from a twin study?

Twin studies allow disentanglement of the shared genetic and environmental factors for the trait of interest. Researchers can estimate the proportion of variance in a trait attributable to genetic variation versus the proportion that is due to shared environment or unshared environment.