What Are The Genetics Of Left-handedness?

It's not a simple dominant-recessive gene thing.  In fact, it's very difficult to predict whether one's offspring will be left- or right-handed.

A left-handed parent is more likely than a right-handed parent to have left-handers.  And left-handed women are more likely than left-handed men to have left-handed children.  But it's no guarantee.

There's more than one gene involved in determining which hand is dominant. Some of the relevant genes probably are just inherited, you either get them and they influence you or you get them and they are dominated by another gene.  But otherwise, it looks like environment, espeically in the womb, can trigger (or not trigger) left-handedness.  We know this from studies of identical twins.  It is very common for these genetically identical individuals to have different preferences in hand dominance (one is right-handed, one is left-handed).

First-born children, children born to older mothers, and children born 6th or later in the family are more likely to be left-handed, too.