Answer:
When both parents have the genotype HbA HbS, they are carriers of sickle cell trait but do not have the disease themselves. In this case, each parent carries one normal hemoglobin gene (HbA) and one sickle hemoglobin gene (HbS).
Step-by-step explanation:
Among their offspring, there is a 25% chance of having the HbA HbA genotype, which results in individuals with normal hemoglobin and no sickle cell trait. These individuals are unaffected by sickle cell anemia and are generally resistant to malaria.
Therefore, 1 out of 4 or 25% of the children from two parents with the HbA HbS genotype is expected to be unaffected by sickle cell anemia and resistant to malaria.
It's important to note that while these individuals are less susceptible to malaria due to their heterozygous genotype (HbA HbS), having two normal alleles (HbA HbA) doesn't confer resistance to malaria. This resistance is primarily associated with the presence of the sickle cell trait.