Twins and supertwins. Philadelphia: J. An older, but complete survey of the history, biology, and psychology of twins before this became mainstream science. Often includes information that is difficult to find elsewhere. Segal, Nancy L. Entwined lives: Twins and what they tell us about human behavior. New York: Plume. A comprehensive overview of the background, methods, findings, and implications of twin research.
Nine of the sixteen chapters address special topics such as athletic performance, legal circumstances, conjoined twinning, and noteworthy twin pairs. Written by a professor of psychology. Stewart, Ellen. Exploring twins: Towards a social analysis of twinship. London: Palgrave Macmillan.
Addresses the social, societal, and cultural aspects of twinship. Also considers various views of twins from the perspectives of the twins, their family members, and society at large. Draws on sources from multiple disciplines. Wright, Lawrence. Twins and what they tell us about who we are. New York: John Wiley. An account of research concerning genetic and environmental events making MZ twins both alike and different in behavior. The focus is largely, but not exclusively, on separately raised twins.
A very good starting point for work in this area, although more recent publications from the Minnesota Study of Twins Reared Apart should be consulted. Written by a well-known journalist. Users without a subscription are not able to see the full content on this page. Please subscribe or login. Oxford Bibliographies Online is available by subscription and perpetual access to institutions.
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Subscriber sign in You could not be signed in, please check and try again. Username Please enter your Username. 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.
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. 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 years ago, scientists found a powerful way to study the effect of genes vs. 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. 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. 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. In fact, they are in such high demand by scientists that an annual twins fair is held in Twinsburg, Ohio, where scientists set up booths hoping to attract twins to their studies. When scientists first started studying twins, they relied on the basic premise that twins shared either some or all of the same genes, and were raised in similar environments.
They often used twins to study the health effects of certain habits, like smoking or dietary habits, or to see if a certain treatment worked well. For example, back in the s, Linus Pauling theorized that vitamin C could effectively cure the common cold. To test out the idea, researchers recruited sets of identical twins and for days had one twin take a vitamin C supplement and the other take a placebo. But in recent years, twins have been providing even more powerful data.
Identical twins, also called monozygotes, have exactly the same genes. But as they grow, identical twins, while they still often look strikingly the same, can develop subtle differences: One twin grows a few inches taller than the other, or one twin has a distinctively different weight or facial feature that makes it easy to distinguish from the other twin.
But because they share the exact same genome, scientists attribute these differences to the environment. So by studying identical twins, researchers can know with more certainty how the environment interacts with our genes and affects our health and how we look—a concept known as epigenetics. The basic idea behind studying identical twins is that the results can better determine whether a certain trait, illness, or disorder is influenced more heavily by genetics or by the environment.
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