With respect to genetics, the basic principle of how alleles can arise and be shown within the offspring, pattern of inheritance, would be that of incomplete dominance.
The phenotype of the offspring is a blend and or intermediate of the 2 parents, and thus the child would have both alleles, one for straight and curly hair. Knowing that the child is heterozygous in terms of genotype, in this mode of inheritance, he/she would have a different phenotypic character that forms due to the presence of both alleles.
The solution is incomplete dominance.
