Why are twins considered a gift from God to geneticists?

When you walk on the street and see two twins who look exactly the same, wearing the same clothes and even holding the same toys, you will definitely find it very interesting and can't help but take a second look. For the parents of twins, pushing a double baby stroller with twins on a leisurely afternoon may make them happier than driving a Ferrari. In fact, twins are not only a gift from God to a family, but also a great gift from God to geneticists all over the world.

How do twins come about?

Twins are divided into two types: identical twins and fraternal twins. Identical twins are formed by the abnormal division of the same fertilized egg into two embryos. Since identical twins come from the same fertilized egg, they have exactly the same genetic material, so identical twins look very similar, and sometimes even their parents can hardly tell them apart. Fraternal twins, on the other hand, develop from two independent fertilized eggs. Like other ordinary brothers and sisters, they have an average of only 50% similarity in genetic background, so fraternal twins may not look the same, and their gender, etc.

Why are twins considered a gift from God to geneticists?

Because twins share a lot of genes, geneticists have long conducted twin experiments to study the effects of innate (genetic) and acquired (environmental) factors on human diseases and behaviors, and to estimate the heritability of specific diseases and behaviors.

Since identical twins and fraternal twins are born in the same womb and usually grow up in the same family environment, the difference between them is that identical twins are 100% genetically similar, while fraternal twins are only 50% genetically similar on average. If genetics plays a role in a certain disease or trait, we can expect identical twins to be more similar than fraternal twins in this regard. If genetics does not play a role in this disease or trait at all, we can expect identical twins and fraternal twins to be equally similar in this regard.

As for the trait of height, the probability that identical twins have the same height is 95% (that is, the similarity in height of identical twins is 95%). This 95% similarity is due to the genetic similarity of identical twins and the shared environmental factors (such as the same family environment, etc.), that is, the genetic similarity of identical twins + shared environmental factors = 95%, and the 5% difference in their height is due to environmental factors that they do not share (such as they went to different schools).

If the height similarity of fraternal twins of the same sex (because identical twins are of the same sex, it is best to choose fraternal twins of the same sex for research) is 55% (that is, the genetic similarity of fraternal twins + shared environmental factors = 55%), since the average genetic similarity of fraternal twins is only half of that of identical twins. Therefore, the 40% we get from 95%-55% is actually half of the genetic similarity of identical twins in height. Therefore, we can estimate that the genetic similarity of the height of identical twins is 80%, that is, the heritability of the trait of height is 0.8 (the range of heritability estimates is from 0 to 1, and the larger the value, the greater the impact of genetic factors on the specified trait). In addition, we can also estimate that the impact of environmental factors (including shared and non-shared environments) on the trait of height is about 20%.

With the help of the design ideas of the above twin studies, geneticists have been able to estimate the heritability of many human diseases or traits, which has also helped us understand to what extent DNA shapes us. Some of these discoveries are subversive to our cognition. For example, geneticists have found profound influences of heredity in depression, dyslexia, intelligence, academic achievement, divorce and even income levels. The blogger will further interpret the genetic influence of the above phenotypes. In addition, what other diseases and behaviors do you think are also affected by genetics? Welcome to leave a message in the comment area to discuss.

References

1. Genes: The Inheritance of Unequal

2. https://medlineplus.gov/genetics/

3. Tenesa A, Haley CS. The heritability of human disease: estimation, uses and abuses. Nat Rev Genet. 2013;14(2):139-149. doi:10.1038/nrg3377

4. The original materials of some pictures are provided by Servier Medical Art (http://www.servier.com). Please indicate the source when reprinting.