Hitchhiker's thumb, also known as distal hyperextensibility of the thumb is the condition of having a thumb that has a distal phalange that bends backwards in an angle of 90°, it is thought to be inherited in an autosomal recessive manner, however, it presents on genetic disorders that are inherited in an autosomal dominant manner. This condition is benign (when isolated) and does not cause pain or affect the thumb with the trait negatively.

Example of bilateral hitchhiker's thumb.

Specialty-Medical genetics. Symptoms- A thumb that can bend backwards at more than a 90° degree angle. Complications -If it presents as an isolated trait, none

Duration- Life-long. Causes-- May either present as an isolated trait, or be a symptom of a condition such as a hypermobility spectrum disorder

Risk factors--- Family history. Frequency---- 24-36% of the U.S. population

If a person has the genes for this condition, it might present bilaterally, unilaterally, or on none of the thumbs.

More Info: en.wikipedia.org