Down syndrome (trisomy 21), where the prevalence of the simian line is usually reported to be around 40%;
- Edwards syndrome (trisomy 18), where the prevalence has been confirmed to be around 70% - which is higher than in any known human (sub-)population around the world.
Interestingly, in both Down syndrome and Edwards syndrome the simian line is often featured with another (more rare) form of 'fusion' between lines - which manifests through the formation of a 'single interphalangeal crease' in the 5th digit (pinky finger): see picture below.