in our family we have every type of hair you can imagine. I have tonnes of hair and when it is short it is straighter, when I let it grow, it is kinky and a little curl but nothing consistent. My brother has beautiful hair (when he had hair) that looked like it just came out of a braid when it was longer and curlier when shorter.
One sister had ringlets like crazy and about to her mid back. When hers was shorter, about 15 years ago, in one of those wonderful stylish bowl/mushroom cuts it was almost straight. She hated it so much she will never go short again. The baby sister has fine, thin, poker straight hair that won't hold a curl without tonnes and tonnes of product.

My son ended up with super curly hair and lots of it. When he takes off his toque in the winter it stands on end  but when it is shorter, it is a very nice wavy/curly hair. He has now learned that he needs to keep it shorter or he will look like a puffy, curly, fro.