How could 2 people without sickle-cell anemia to have a child with sickle-cell disease?

Genetics. Sickle cell disease must be inherited from both parents. Sickle cell trait (1 sickle gene, 1 normal gene) is usually asymptomatic. 2 persons with sickle cell trait have a 1/4 chance of having a child with sickle cell disease, but the parents are often unaware that they have anything.
Parents have trait. One biological parent would need to have sickle cell trait and the other biological parent would need to have sickle cell trait/ hemoglobin c trait/ or a form of beta thalassemia trait - depending upon the type of sickle cell disease.
Each carries S Trait. Recessive disorders are so named because they are not expressed unless two genes of the same type are carried in the chromasomes. Dominedt genes can show up when only one gene is present. Sickle cell requires 2 recessive genes, one from each unaffected parent. Prenatal testing and genetic counseling are encouraged for populations at risk.