A single transverse palmar crease appears in about 1 out of 30 individuals. So really, it is more common than you think! Sometimes, as it is the case with Down syndrome, fetal Alcohol, and other conditions, this can be one of several markers for such condition.
Medline Plus has some more information on the transverse palmar crease if you want to check it out.
What's interesting is that I know people who have maybe one of these characteristics that do not have Down syndrome.
Yes, many of the characteristic features of Down syndrome are found in people that do not have Down syndrome. Remember, that the characteristic of Ds are simply seen more often, but not exclusive of Down syndrome.
Nichole, for example, does not have a transverse palmar crease.
My ears, for example, sit a little lower on my head.
I know several people that have the transverse palmar crease and do not have Down syndrome.
I have a friend that has tiny little ears that bend slightly at the top and she does not have Down syndrome.
Some of my nieces have the tiniest bridges of the nose.
I have a niece with hypotonia.
Like I said before, these characteristics can be found in the global population, it is just that several of them seem to come together for individuals with Down syndrome.