Sickle Cell Disease (SCD) is an inherited blood disorder that causes red blood cells to become rigid and crescent-shaped. These misshapen cells can block blood flow, leading to pain, infections, and organ damage. Sickle Cell Trait (SCT) is a carrier state where a person carries the gene but does not have the disease. Understanding the genetic transmission pattern explains whether a child will inherit the trait if only one parent has it.
The Basics of Sickle Cell Genetics
Sickle cell conditions are determined by the genes responsible for producing hemoglobin, the protein in red blood cells that carries oxygen. Everyone inherits two copies of this gene, one from each parent. The two main types of hemoglobin genes are A (normal hemoglobin) and S (sickle cell hemoglobin).
The combination of these genes determines a person’s status. A person with two normal genes (AA) is unaffected. An individual who inherits one normal gene (A) and one sickle gene (S) has the Sickle Cell Trait (AS) and is a carrier.
Sickle Cell Disease (SCD) develops only by inheriting two sickle cell genes (SS), one from each parent. Since the condition is recessive, having a single S gene results in the carrier trait (AS) but not the full disease (SS).
Inheritance When Only One Parent is a Carrier
If one parent is a carrier (AS) and the other is unaffected (AA), the child’s outcome is determined by which gene the carrier parent passes on. The parent with the trait (AS) can pass on either the normal A gene or the sickle S gene, while the unaffected parent (AA) can only pass on the A gene.
In this scenario, the child cannot inherit Sickle Cell Disease (SS) because receiving two S genes is impossible. There is a 50% chance the child will inherit the normal A gene from both parents (AA genotype), meaning they are unaffected.
There is an equal 50% chance the child will inherit the sickle S gene from the carrier parent (AS genotype), resulting in the Sickle Cell Trait. If only one parent is a carrier, the child has a 50% chance of inheriting the trait and a 0% chance of inheriting the disease.
Living with the Sickle Cell Trait
A child who inherits the Sickle Cell Trait (AS) is typically healthy and does not experience the symptoms associated with Sickle Cell Disease (SCD). The presence of sufficient normal hemoglobin (A) prevents the red blood cells from consistently assuming the sickle shape. People with the trait usually live normal lifespans without health complications related to their carrier status.
While generally asymptomatic, rare complications can arise under extreme physiological stress. These conditions include severe dehydration, intense physical activity, or exposure to very high altitudes with low oxygen. Under such circumstances, the red blood cells can temporarily sickle, which may lead to issues like splenic infarction or muscle breakdown, known as rhabdomyolysis.
For the vast majority of individuals, having the trait is not a medical illness and does not require continuous medical intervention. The trait cannot progress into the full disease later in life. Awareness allows for simple precautions, such as maintaining hydration and avoiding extreme physical exertion in high-risk environments, particularly for athletes or military personnel.
Next Steps: Genetic Screening and Counseling
Newborn screening provides diagnosis for parents concerned about their child’s status. In the United States, all newborns are routinely screened for sickle cell conditions shortly after birth using a simple blood test. This testing identifies if the baby has the trait (AS), the disease (SS), or is unaffected (AA).
If screening identifies a child with the Sickle Cell Trait, genetic counseling is often offered to the family. Counselors explain the test results, clarify the difference between the trait and the disease, and discuss implications for the child’s future, including reproductive planning.
For couples planning future pregnancies, carrier screening for both parents is recommended if their status is unknown. Knowing both parents’ statuses is important. If both partners are found to be carriers (AS), the risk of having a child with Sickle Cell Disease (SS) rises to 25% for each pregnancy.