Sickle-cell disease is a debilitating disease that results from being homozygous recessive for the "sickle-cell allele" of a hemoglobin gene. Why is the allele present and even common in many human populations?

Heterozygote advantage

Explanation:

The sickle cell allele in a heterozygote state confers a kind of resistance to malaria in areas where the disease is endemic while it is deleterious in homozygous condition. Hence, this heterozygote advantage ensures that the recessive allele is present and quite common in the population.

who knows

explanation:

children tend to take more after their grandparents more than their parents. it’s the dna you received that was passed down to you somewhere in your family. maybe one of your grandparents had/has straight hair and you might have gotten it from them.