Simply Slender

Why do I only have one clubbed thumb?
My brother and I both have what is called a clubbed thumb, only he has 2 and I only have 1. This runs in my family as my aunt has 2 clubbed thumbs.
A clubbed thumb is simply a shorter thumb with a wide, short nail bed.
I’d like to know what causes this to occur in one’s genetics and how its possible that I only have 1 clubbed thumb. My other thumb is long and slender, matching my fingers.
I just want some input and I’d also like to hear about anyone else who has this kind of thumb.
My daughter has one clubbed thumb, it looks like a big toe. She does her nails, that one too, puts those little diamond things on it, jokes about it, loves it because it’s part of who she is.
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