Tongue rolling genetics is often taught in the classroom as a simple demonstration of Mendelian inheritance. However, the picture is not as clear-cut as some of the text books would have us believe. The ability to role the tongue is not purely genetic; the environment plays a part too.
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Is Tongue Rolling Genetic?
Yes, and no. The text book version of tongue rolling genetics goes something like this;
Some people can roll their tongue into a tube, and others can't. The ability is due to the dominance or otherwise of tongue rolling genes.
Many of our genes come down to us in pairs - one from mum, and one from dad. Each gene has two versions. The scientific language for a version is allele. So each gene has two alleles.
Tongue rolling is taught as being a two allele trait - the allele for rolling (with the symbol R) being dominant over the non-rolling allele (with the symbol r).
What this means is that if one of your parents has the dominant tongue rolling gene and they pass it on to you - then you will have the ability to roll your tongue. A dominant version of a gene always wins out over a recessive version. However, the picture is not so simple.
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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.
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Further Tongue Rolling Possibilities
Or it could be that in order for a person to able to roll their tongue they must also possess a modifier gene. This is a gene that is able to turn on or turn off gene expression. So for tongue rolling it has to be in the "on" position. Now let's take two parents who cannot roll their tongues. They have the dominant gene but the modifier gene is in the "off" position so they can't roll their tongues, yet they could still have children with the tongue rolling ability, if they don't pass on the modifier gene.
And there's another layer of explanation. Genes make proteins, and perhaps there is a gene lurking somewhere in the genome and it has the ability to alter or block the action of tongue rolling proteins created by the tongue rolling genes. It changes them in such a way that it affects the ability to tongue roll.
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Tongue Rolling is not so Simple
The above list is not exhaustive, but it gives you some idea of the complexity involved in something that was supposedly a simple Mendelian trait. In genetics, as in the whole of biology, it is not always so simple, and the scientist always has to work hard to tease out the cause or causes of the effect.