A complete fusion between the heart- and head line is only seen in the concept of the simian line; this unique characteristic is missing in the concept of in respective the Sydney line, extended heart line, and Suwon crease (in the latter is a 2nd head line required to create a fusion between only the upper head line & the heart line).
The simian line is known to have the highest prevalence in the so-called 'trisomy syndromes', which represent a group of genetic disorders featured with an extra chromosome that creates 'trisomy' in one of the 23 chromosome pairs. The two most common trisomy syndromes represent in respective: