Good question.
The answer may lie in the way that hair-type genes are inherited.
Hair-type doesn’t follow the nice and simple pattern of inheritance. Hair-type follows a type of inheritance pattern known as “incomplete dominance”. Like the earlobe gene, there are two versions of the hair-type gene, curly (noted as C) and straight (noted as s). The incomplete dominance refers to the fact that if you have one of each version of the gene, you get a mix of the two or, in this case, wavy hair. So for hair type, CC gives curly, Cs gives wavy and ss gives straight hair.
Remember the scare. Have a friend who lost all hair due to chemo. When it came back in it was as curly as BO’s