If both parents are carriers of the sickle-cell trait (heterozygous for the sickle-cell allele), they have a 25% chance of having a child with sickle-cell anemia (homozygous for the sickle-cell allele) with each pregnancy. This is because during reproduction, each parent can pass on either the normal hemoglobin allele (HbA) or the sickle-cell allele (HbS) to their offspring. If both parents pass on the sickle-cell allele, the child will have sickle-cell anemia.
In short, a sickle-cell anemic child can result from both parents being carriers of the sickle-cell trait and passing on the sickle-cell allele to their child.
