What are the possible genotypes of the parents of a child who has wavy hair (Hh)?

Well supposing the H=wavy and h=straight the parents could be either HH (zygous) or Hh (heterozygous) uming wavy hair is the dominant trait but if straight hair is the dominant trait S= straight and s=wavy the parents would have to be both ss (zygous) or one ss and one Ss (heterozygous). You could do a punnet square on that.