TBS is caused by a harmful genetic change (mutation) in a gene called SALL1. The SALL1 gene has instructions for the body to form certain tissues and organs such as the hands (particularly the thumbs), ears, anus and kidneys. Humans should have two working copies of the SALL1 gene, but people with TBS have one copy of the gene that does not work properly.
There is also evidence that this condition could be caused by a mutation in the DACT1 gene, but a mutation in the SALL1 gene is the more common cause.
TBS is inherited in an autosomal dominant manner. Dominant genetic disorders occur when only a single copy of a non-working gene is necessary to cause a particular disease. The non-working gene can be inherited from either parent or can be the result of a changed (mutated) gene in the affected individual. The risk of passing the non-working gene from an affected parent to an offspring is 50% for each pregnancy. The risk is the same for males and females.