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I have a biology book that states that whether a person will be left-handed or right-handed is determined by the genetic constitutional makeup.

Is this true? As far as I knew, these traits depend on how a person is trained as a child: every child has a tendency to use the left hand first when he starts to write, but if the parents teach him, then it will change to the right hand.

What is the truth?

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    $\begingroup$ Not true about the writing. If a normally left-handed person is forced to write with their right hand (as many were in my youth) the result will be a clumsy kid. $\endgroup$ – jamesqf Mar 26 '17 at 18:17
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Handedness has, like many traits, a non zero heritability. You now should read the post Why is a heritability coefficient not an index of how “genetic” something is? to understanding what heritability mean.

In short, yes part of the variance in handedness seen in the human population is due to genetic variance.

You will note that footedness is highly correlated with handedness (wiki > footedness).

Note also that today, in the western world at least, it is generally considered unhealthy for the kid to force him/her to use its other hand (as the catholic church used to do). I am not sure what are the evidence for that but it is a totally different question.

References

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