Describe marshall nirenberg's contribution to determining the role of rna in genetics. any little info ! : -)

In 1959, he demonstrated that mRNA is significant in protein synthesis. By contributing to the central dogma, he showed that just like DNA, mRNA can be used to determine the genetic code. His experiment involved a cell whose DNA was denatured but injected with mRNA and showed that protein synthesis proceeded.  

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.