Someone who has a thumb which bends backward when extended is said to have a hitchhiker's thumb. This is a genetic trait, and it does not interfere with the thumb's normal functions. Hitchhiker's thumb is also not linked with any other genetic conditions; it is simply an interesting phenotype, akin to people who can curl their tongues. To see if you have a hitchhiker's thumb, make a fist and extend your thumb. If you notice a significant bend, you have inherited this trait.
Hitchhiker's thumb is a recessive trait, which means that people must inherit the gene from both parents for it to manifest. People can also carry the gene without exhibiting the trait, by inheriting the gene from one parent only. Recessive traits are particularly interesting because they can appear to flit in and out of a family with no apparent reason, thanks to the fact that they can lurk in generations of genes before manifesting.
Some people with a hitchhiker's thumb are capable of bending the joint in the thumb to extremes which may look uncomfortable to people who lack this genetic trait. You may also hear hitchhiker's thumb referred to as “hyperextension of the thumb” in a reference to this. There is no particular advantage or disadvantage of having a hitchhiker's thumb, and it certainly does not predispose people to hitchhiking.
Having a hitchhiker's thumb is not the same thing as being double-jointed. People who are double-jointed display a trait known as hypermobility, which allows them to move their joints much further than is normal. In this case, all of the fingers of the hand can be bent and twisted in interesting ways, in addition to the thumb.
The study of recessive genetic traits can be very interesting, because it can reveal intriguing information about family and regional histories. Some teachers like to use the hitchhiker's thumb in classroom exercises which demonstrate the basic principles of genetic inheritance, showing students what a hitchhiker's thumb looks like and then encouraging them to collect data from classmates. In some families, such traits may become topics of discussion and commentary, with children being told that they have “inherited Aunt So-and-so's thumb” to give them a deeper connection with the rest of the family.