What is a Hitchhiker's Thumb?
I started taking piano lessons when I was six years old. Although I had long fingers for my age, I always had an issue with the keys in the middle of the keyboard. My thumbs never quite hit them the right way.
My thumb weirdness continued into other activities, with my hands not always working quite as I wanted. I was in high school before I learned why my thumbs seemed so different from everyone else's.
I had hitchhiker's thumbs!
How Does a Hitchhiker's Thumb Work?
Hitchhiker's thumb is a layman's term for distal hyperextensibility of the thumb.The joint closest to the end of the thumb is called the distal joint. In most people, the distal joint of the thumb allows the thumb to straighten and to bend forward (toward the palm of the hand). With a hitchhiker's thumb, the distal joint hyperextends, allowing the thumb to go beyond the straight position and point toward the back of the hand.
Basically, my thumbs not only bend forwards, but they also bend backwards. In my case, both bend ninety degrees backwards. So THAT's why using scissors was always so uncomfortable!
There are variations in the hypterextension of a hitchhiker's thumb. Some people can bend their thumbs backward but have limited flexibility going forward. Others, like myself, have a full range of motion in both directions.
A Hitchhiker's Thumb In Action!
A Recessive Trait
Let's address a bit of genetics when discussing how a hitchhiker's thumb happens, and why it is so rare.
As part of your genetic makeup, there is a gene that determines whether your thumbs are hyperextendable or not. We'll call this the "bendy thumb gene", for lack of a better term.
For each bendy thumb gene, there are multiple alleles. Alleles are the variations of a given gene that can exist. For example, one allele of the "bendy thumb gene" may produce a perfectly straight, "normal" thumb extension. Another allele of this gene may be a thumb that can bend backwards and forward at ninety degrees each way. The "bendy thumb gene" has a spectrum of alleles, from perfectly straight to bending all the way and everything in between.
Genes are inherited from your parents, and one of each type of gene is inherited from each parent. So, the each of us has two genes (one from Mom and one from Dad) that determine your thumb bending ability. Depending on what alleles you receive from your parents, your thumb could fall anywhere in the spectrum.
A hitchhiker's thumb is called a recessive trait. There are two general types of traits - dominant and recessive. A dominant trait comes from a dominant allele, which means it overpowers any other allele with which it is paired and is the allele that is displayed. A straight thumb is dominant, so anyone with a straight thumb allele, regardless of their other allele, will have a straight (non-hitchhiker's) thumb.
A recessive trait comes from a recessive allele. In fact, in order for a recessive trait such as a hitchhiker's thumb to exist, you must receive a recessive allele of the "bendy thumb gene" from BOTH parents. In other words, it takes two recessive alleles to get a hitchhiker's thumb.
How Do You Get A Recessive Trait?
Let's look at an example of how this works. Below is a Punnett Square, which is the accepted method for displaying the probability of having a specific trait.
Let's represent a straight thumb (the dominant allele) with a capital "A", and the hitchhiker's thumb (the recessive allele) with a lowercase "a". The left column represents which allele you received from your mother. The top row represents which allele you received from your father. the cells in the middle represent your genotype, or the genetic makeup you have with respect to the "bendy thumb gene".
Father - Dominant Gene (A)
Father - Recessive Gene (a)
Mother - Dominant Gene (A)
Mother - Recessive Gene (a)
In the above example, any outcome that has a capital "A" (the dominant allele), will have a straight thumb. This is the case for the outcome with the "AA" genotype, as well as both cases of the "Aa" genotype. In the latter, the dominant (straight thumb) gene will overpower the recessive one. The only outcome above that will display the hitchhiker's thumb is the "aa" combination of alleles.
In my particular situation, my father displays the hitchhiker's thumb, so his genetic makeup is "aa". I display the hitchhiker's thumb, so mine must also be "aa".
My mother does not display the hitchhiker's thumb. However, in order for me to display it and have the "aa" genotype, I must have inherited a recessive "a" allele from her as well. So, she must have the "Aa" genotype, with the dominant gene masking the recessive one.
Hitchhiker's Thumb Tricks!
Not many people have seen a hitchhiker's thumb like mine, that can bend backward ninety degrees. So I have a lot of fun playing tricks with it!
I can always scare people (especially kids) by pretending my thumb is broken. I push down on a table and bend it back, acting like I broke it. Guaranteed to freak kids out!
My thumbs are also a great conversation starter. There aren't many people with hitchhiker's thumbs, so most people are surprised at my "ability." After the initial gross-out, it makes for great conversation.
I'm also the grand champion of thumbwrestling! No one can pin me because I can bend my thumb back out of their reach. Ahhh, the perks of a crazy recessive trait!
I've grown to love my thumb weirdness, and am actually a little sad that neither of my children inherited it. It's a bond I will always share with my father and his mother, who both had the same thumbs as I do.