Answer: <u>Both parents are heterozygous carriers</u>.
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.
Marine mammals breed by having coitus
Hope this helps :)
The exchange of oxygen, food, and waste between mother and fetus occur in the placenta.
