My aunt has 3b hair. I am not sure if her exhusband had straight, wavy hair. Both have extremely thick hair. My cousin was born with straight, thin, baby fine hair. Her hair was so thin that her scalp was visible with certain styles and my aunt would keep her hair into a chin length bob because anything longer didn't look good. Once puberty hit, my cousin became a 3a. However, we have all found it strange that my cousin has thin hair since both her parents and their families have such thick hair. Genetics are strange, and I second the co-dominance theory.