Meeting twins you might be dazzled by their similarity or surprised by their differences. Scientists share the same fascination; both their resemblances and differences can help us understand psychiatric disorders.
Why do some twins look more similar than others?
Two twins that look alike generally do so because of the high number of genes they share. Due to a ‘splitting up’ of a newly fertilized cell quickly after it came to be, the two resulting babies carry the exact same set of genes. Because of this, identical (or monozygotic) twins will have an (almost) indistinguishable physical appearance.
However, in other cases twins might not resemble each other that closely. Usually, this is due to the fact that they did not come from the same fertilized egg. Instead two different eggs got fertilized at the same time! They inherited a close but still different group of genes. Essentially, they are like any brother or sister; they are just born on the same day. These are commonly referred as dizygotic twins, and they share on average 50% of their genes.
Similarities between twins are not only explained by the genes they have in common, but also by the life experiences they share. For example, they may have very similar daily routines (e.g., diet, sports, work), because they grew up in the same house. This and other environmental factors can also explain how similar they look and sound.
By examining differences and similarities between both types of twins we try to piece together how important genes and environment are in the development of several characteristics.
How studying twins allows us to understand ourselves: twin models!
To understand how helpful is to study twins, let’s imagine you gather a group of twins and measure their height. Now normally, compared to other people, you are most likely to be of a similar height as a brother or sister of the same gender. When looking at the height of monozygotic and dizygotic twins, you would expect monozygotic twins to have an even more similar height than dizygotic twin pairs, since monozygotic twins share the same genes, along with the same environment, while dizygotic are less genetically similar (but share the same environment).
With these assumptions, we can build twin models: these models can estimate how much height is explained by their genes, and how much is due to the environment they both share1.
Why are twin studies so relevant?
The conclusions we take from twin models can be generalized to all of us, and to the different health conditions in a population. For instance, you can use the same principles in twin models to estimate the causes of disorders and their symptoms. You can study schizophrenia, and test how genes and the environment can separately explain its symptoms, or how these can change structures and functions in the brain.
Needless to say, twin studies have been vital in a lot of what we know about genes and environmental differences today. Further, these studies helped in the discovery of particular environments or genes that are likely to explain psychiatric disorders. Hopefully they will continue to be helpful and contribute even more in the future!
By João Guimarães, edited by Annelies van Nuland
Rijsdijk, F.V. & Sham P.C. (2002), Analytic approaches to twin data using structural equation models, Briefings in Bioinformatics, 3(2), 119–133