Understanding Sickle Cell Anemia Inheritance: A Simple Explanation

Sickle cell disease (SCD) is an autosomal recessive disorder of the β-globin gene (HBB) on chromosome 11.

Genotypes & Phenotypes

Genotype	Hemoglobin	Phenotype	Terminology
HbA / HbA	Normal adult (α₂β₂)	Unaffected	Homozygous normal
HbA / HbS	~60% HbA, 40% HbS	Sickle Cell Trait (carrier)	Heterozygous. Usually asymptomatic. Malaria protection.
HbS / HbS	HbS only (α₂β₂ˢ)	Sickle Cell Anemia (SS)	Homozygous affected. Severe disease.
HbS / HbC	HbS + HbC	HbSC Disease	Compound heterozygote. Milder than SS.
HbS / β⁰-thal	HbS + no β	Sβ⁰-thalassemia	Similar to SS.
HbS / β⁺-thal	HbS + reduced β	Sβ⁺-thalassemia	Milder.

Inheritance Patterns

• Two Trait Carriers (AS × AS): 25% AA (unaffected), 50% AS (trait), 25% SS (disease) — each pregnancy.
• Trait × Unaffected (AS × AA): 50% AS, 50% AA. No disease.
• Disease × Trait (SS × AS): 50% SS, 50% AS.
• Disease × Unaffected (SS × AA): 100% AS (trait).

Population Genetics

High HbS frequency in malaria-endemic regions (balancing selection). Carrier rates: West/Central Africa 10–25%, African Americans ~8–10%.

Screening & Counseling

• Newborn screening (universal in US) — detects HbS, HbC, HbF patterns.
• Preconception/prenatal carrier screening (hemoglobin electrophoresis, HPLC, DNA).
• Genetic counseling for at-risk couples: options include prenatal diagnosis (CVS/amnio), PGD-IVF, adoption, preparation.