What is Hitchhiker's Thumb?

The human body is full of many unusual quirks, including tongue rolling, widow's peak hair, and dimples. To that list can be added hitchhiker's thumb, a congenital recessive condition where the thumb can bend backwards to an angle of almost 45 degrees. It is not painful, and does not interfere with the other digits. It is purely a fascinating autosomal recessive phenotype.

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Being an autosomal recessive condition means that a child has to inherit two copies of the gene for hitchhiker's thumb - one from each parent.

Each gene generally comes in two versions, known as alleles, and an allele can either be dominant or recessive. And an inherited characteristic or phenotype (such as hitchhiker's thumb) is known as a trait.

In genetics a dominant allele is expressed with a capital letter and a recessive allele is represented by lower case letters.

So let's consider the body contortion hitchhiker's thumb. First of all in terms of thumb position, a straight thumb is the dominant allele and expressed with the upper case S. Hitchhiker's thumb is the recessive allele and expressed by the lower case s.

Then, you have to bear in mind that a genotype (which creates the phenotype) is made up of either - two dominant alleles, two recessive alleles, or one dominant and one recessive allele.

Genotype

A person with ss genotype will have hitchhiker's thumb - there are two recessive alleles

A person with Ss genotype will have a straight thumb, but will also be a carrier of hitchhiker's thumb

A person with SS genotype will have a straight thumb, and will not be a carrier of the condition.

• When both parents are unaffected carriers of hitchhiker's thumb, there is a 25% chance that their children will inherit the condition, a 50% chance that they will be unaffected carriers, and a 25% chance that they will be unaffected non-carriers.

• When only one of the parents is an unaffected carrier, there is a 50% chance that their children will be unaffected non-carriers, and a 50% chance that they will be unaffected carriers.

• If one parent has hitchhiker's thumb and the other is not a carrier then each child would be an unaffected carrier of the condition.

• If one parent has hitchhiker's thumb and the other is an unaffected carrier then there is a 50% chance that a child will have hitchhiker's thumb.

To see if you have hitchhiker's thumb, just make a fist, and extend your thumb, and if you see a noticeable bend backwards then you have the condition. Then, read on to see if you have the characteristics of other genetic anomalies.