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Has there been any definitive research about handedness being genetic? Also, why is right-handedness clearly dominant in humans? I'm interested in evolutionary theories, as well as any molecular explanation (if known).
There is a lot of information at OMIM - too much to summarize here - regarding the genetics of handedness and links to which hemisphere of the brain dominates (in an individual), to schizophrenia (slight association with non-right-handedness), and to hair whorl patterns on the scalp. References are provided at the above link. Basically, hand skill appears to be a complex multifactorial phenotype with a heterogeneous background.
OMIM (online mendelian inheritance in man) is a good example to explain how complicated heritability usually is. Simple Mendelian traits, where smooth or wrinkly peas will have wrinkly offspring, allow us to segregate individuals with pure dominant / heterologous and pure recessive traits. When we have done so the chance of inheriting the trait can be predicted precisely (i.e. 25/50/25%).
The number of simple mendelian traits which are observed in an individual are pretty doggone rare. Even things that are dominant like dark hair are usually more complex. I'm of 100% japanese descent, but my son was born with blonde hair and it looks like it won't go darker than mousy brown - black hair is not really dominant, he will have a color between my wife's and mine.
I also need to make a distinction between mendelian traits, which are observed in the individual organsim as opposed to specific DNA variants like a single site mutation (A->T, C->T etc) which are usually mendelian.
The rarity of mendelian inheritance is because there are so many checks and balances in our gene networks that no single gene is solely responsible for any function. Medical research has discovered many single mutation (Mendelian) diseases, but they are very rare in practice. There are single mutations that cause diabetes like systems (called MODY diabetes) for instance, but more than 99% of diabetes are not caused by these mutations. A single mutation advantage is quickly adapted so that single mutations cannot nullify the advantage.
Common mutations that cause diseases or other disadvantages are likewise relatively quick to disappear through adaptations.
I once saw the old text bound copy of "Mendelian Inheritance in Man" It was not a small book, but it seemed to me that when I paged through it, when you look at traits like 'handedness'. there were only a few hundred simple mendelian traits identified. Most of what we are is the result of the interplay of several or even many of our ~30,000 genes.
For an example of how this works, see this article about what happens to new fly genes.