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I am confused by the twin study 'Concordance of Monozygotic and Dizygotic twins for traits.' My questions arose from 3:17-4:17 of the video

And this is the related data I have referred

I have the following questions:

1 Is the dizygotic twins also separated in different families? If true, didn't the DZ group lack the Control Variables of 'same environment 'or'identical genes'? Or should we make the '50% identical genes' as control variables?

2 why do we need Dizygotic-Twin groups in the experiment, if we need to find the environmental influences why not just find a pair of unrelated individuals and set them at a same family (environment)? Alternatively, is it necessary to compare the monozygotic groups with dizygotic groups instead of any others that aren't dizygotic twins?

Snake
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My interpretation of the first question is why do traits determined by the environment appear to be equally similar among monozygotic and dizygotic twins in these twin studies. The studies he is talking about look at twins that are separated and adopted in to different families. These different families provide a different environment for either twin, and with enough samples, you can compare similarity between dizygotic and identical twins. So imagine you measure the trait of interest (height) in all individuals, and find that identical twins more closely resemble each other than dizygotic twins do. This would indicate that height is somewhat genetically determined - the more genetically similar pairs were more phenotypically similar.

Why use mono- and dizygotic twins individuals to test environmental effects? Comparing mono- and dizygotic twins is a good experimental design - it removes the effects of being a twin (nutritional environment in the womb may differ if you use monozygotic twins vs single born siblings), effects of year and season of birth, effects of womb environment among the two single born individuals, and in one group you have zero genetic variance, while both groups should have the same amount of environmental variance. Read here about heritability, he gets the explanation wrong in the video, heritability is not a measure of how genetic something is, it is how much of the phenotypic variation ($V_P$) is explained by additive genetic variation ($V_A$).

$$h^2 = V_A/V_P$$

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rg255
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  • Thanks for your answer ,but I still have some questions about the first part of your answers. If there is a TRAIT determined by environment , both the identical twin group and fraternal twin group have 100 samples in each of two families. After the research , in identical or fraternal group , it is found that both families have 30 of 100 samples that have TRAIT,so 30% people have same trait in the group.My question is why the difference of both environments can cause that 30% same TRAIT, and also equally similar to those in the other group. – Snake Jun 21 '16 at 10:50
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    If the trait was genetic then the trait would be more common in the group that is more genetically similar (the identical twins), but if the trait is entirely environmental (and assuming both groups experience the same environmental variation - requires large unbiased sampling effort) then there is no reason for either group to differ in the frequency of the trait. – rg255 Jun 21 '16 at 11:01
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    For example; if trait is caused by exposure to chemical-A and 30% of people are exposed to chemical-A then both groups should have a trait frequency of 0.3 (or 30%). However, if trait is determined by genetics, then identical twins are identical for trait, i.e. if one has trait, the other in the pair also has trait, while in the non-identical twins the presence of trait in one twin is not as informative about the other in the pair (the second twin may have trait, but not necessarily) – rg255 Jun 21 '16 at 11:05