What will happen to the frequency of the recessive allele for the HbS gene when there is an outbreak of malaria?

The answer is the frequency of the recessive allele will increase.

HbS gene is a variant of the β-globin gene responsible for sickle hemoglobin. Sickle-cell anemia is a recessive disorder caused by the presence of two recessive alleles. But, when there is an outbreak of malaria, frequency of the recessive allele for the HbS gene will increase. This is because in an area with malaria, heterozygous individuals (with one dominant allele and one recessive allele) have an advantage. These individuals will have both normal and sickle hemoglobin. But pathogen that causes malaria affect only normal hemoglobin, so heterozygous individuals are more resistant to malaria.