Confused About Twin Studies?

Here are three examples of how identical and fraternal twins are useful in determining the role of genes and environment in creating variance in a trait:
  

I. For a completely genetically-determined trait, such as eye-color: identical twins will always have the same eye-color, some fraternal twins will have the same eye-color and others will not.
II. For a predominantly genetically-determined trait, such as height: Identical twins will usually be the same height, or close to it. Fraternal twins will vary more. The variation between identical and fraternal twin sets indicates that height is mostly determined by genes, but that social/environmental factors—nutrition, for example—can play a role.
III. For an environmentally determined trait, such as religious observance, both identical and fraternal twin pairs will show quite a lot of variation, and there will be the same amount of variation within the identical twin pairs as within the fraternal twin pairs.