According to the law of incomplete dominance, " When a cross is made, neither of the two alleles has complete dominance over the other allele, and the resulting offspring will have a mix of traits of the parents' phenotypes."
Curly and straight hair are both incompletely dominant alleles of the same gene.
A person who carries two homozygous curly hair alleles (hh) will have extremely pure curly hair.
Whereas straight hair is the result of two homozygous straight hair alleles (HH).
When a cross is made between homozygous curly hair alleles (hh) and homozygous straight hair alleles (HH), the progeny will inherit the mixed traits of both the phenotypes, i.e., the progeny will have wavy hair.
Hence, it is concluded that curly hair has an incomplete dominance effect.