Rolling the Tongue Inheritance
Studies, and simple observation have revealed that the ability to roll a tongue must be more than just human genetics. For example parents who cannot roll their tongues can have children with the ability, but the clincher is identical twins. Research has shown that they don't necessarily share an ability to tongue roll (Matlock, P. 1952. Identical twins discordant in tongue-rolling. J. Hered. 43: 24).
The reason why this is such an interesting development is that identical twins have identical DNA, so if the trait was purely genetic and one twin could roll their tongue, then so should the other. But this is not always the case. So what is happening?
Well it could be that the twins do in fact possess the dominant gene for tongue rolling, it's just that it is not expressed. Perhaps it needs some environmental cue or trigger. This can be quite common in genetics.
For example, there is a heart condition called dilated cardiomyopathy which causes the heart to enlarge. There are many ways that a person can contract this type heart disease, but one of them is to be genetically predisposed to the condition, and the causative genes only kick into action in response to an environmental cue, such as a virus.