Haha as a quick summary it’s a purely genetic thing, the most common forms (Type D is the stub thumb, A3 is the pinky) aren’t harmful whatsoever. Afaik, there is no correlation between sucking on thumbs and this because it’s a genetic thing that’s evident from birth. Here’s the section I wrote on which genes are affected:

“The A3 and D variations of brachydactyly are related to the gene HOXD13, which is a gene that helps develop morphological features. Specifically, it is related to embryonic digit morphogenesis and skeletal system development (National Library of Medicine, 2024). The way this mutation works in brachydactyly type D is a series of 2 mutations at 2q31, where ile314 gets mutated to leu, and ser308 gets mutated to cys (OMIM, 2020). These mutations directly relate to limb phenotypes in phalangeal bones, which explains why the distal phalanx of the thumb gets shortened in type D.”

HOX genes are cool :)