One parent has the sickle cell disease gene, and the other parent does not. Which best describes the child’s chances of getting sickle cell disease?

1.impossible, because the child would have to inherit two copies of the gene
2.unlikely, because few children can be affected by inheriting one copy of the gene
3.possible, because the child would only have to inherit one copy of the gene
4.likely, because the child can be affected by inheriting two copies of the gene

Sickle cell disease is a genetic disease which is autosomal recessive in nature. By autosomal recessive means a person will be affected by the disease only if it will have 2 copies of the disease causing gene at a particular locus in the autosome. If only one copy of the disease causing gene is present then the person will be carrier for the disease but phenotypically the person will be normal.

In the question, it is clearly mentioned that only one parent has the gene which causes disease while another one does not have the gene. But the child can inherit the disease only if another parent who does not have the disease is either carrier or diseased because in autosomal recessive disorder two copies of the diseased gene must be present. That is why it is impossible that their child will have the disease

The allelic combination is shown as under:

Here, 'S' denotes normal allele of the gene while 's' denotes diseased allele of the gene.

Diseased x Normal

Parental: ss SS

F1 Ss Ss Ss Ss

So, it is clear from above that in this case, all the children born from these two parents will be phenotypically normal but they will be carrier for the disease.