Achild has sickle-cell disease (ss) but neither parent is ill. what must be true of her parents’ genotypes? one parent has two recessive alleles. both parents are heterozygous carriers. the father is heterozygous; the mother is normal. the mother is heterozygous; the father is normal.

Both parents are heterozygous carriers.

Explanation: Since sickle cell disease is a recessive trait, it means the affected person will have "homozygous recessive" state of genotype only. Here parents are normal, which means they are carriers. So their genotype would be Ss, heterozygous.

Heterozygous parents only will have 25% chances of having their children affected.

Option 1 is incorrect, because if one parent has two recessive alleles, it means he/she is affected and this contradicts the situation mentioned in question.

Option 3 and 4 are incorrect because it is mentioned in question that both parents are normal.

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

explanation: