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Most people have five vertebrae in their lumbar (lower back) region, which are named L1 to L5. However, some people possess an additional lumbar vertebra located below the L5. This extra vertebra, known as the L6, is called a transitional vertebra. About 10 percent of adults have some form of spinal abnormality caused by genetics, and a sixth lumbar vertebra is among the most common of these abnormalities.

Question- What is the current explanation to why some humans have developed this extra vertebra in the spine?

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    $\begingroup$ What do you mean by genetic link? Are you looking for the names of the genes which variation explain variation in number of vertebrae? Or are you looking for the fraction of the phenotypic variation that is explained by genetic variation (namely, the heritability)? $\endgroup$
    – Remi.b
    Dec 6, 2015 at 10:08
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    $\begingroup$ Your phrasing suggests some confusion on these concepts and you might want to read on the concept of heritability (even if it may seem unrelated to your right now). On this post I linked 4 other posts (and the wikipedia article) that talk about heritability. $\endgroup$
    – Remi.b
    Dec 6, 2015 at 10:08

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