Cats without eyebrows?

I noted that my now deceased tonkinese did not have eyebrows - sparse fur from the ear down to above the eye, and have now adopted a black cat who has the same eyebrow pattern going on. He's black with gold eyes, not purebred as he's got some stray white hairs. Any ideas what breed/s this eyebrow trait comes from? Since my tonk was 1/2 burmese I assumed it came from there, but I can't tell from the pics I've viewed online.

All my cats have the sparse hair too, 4 dsh & 3 dlh. I'm looking at my sisters 5 dsh, they all have it too. I guess it's normal.