HBB gene — Three Level Genetics

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HBB

The β-globin gene (HBB) carries a single point mutation (Glu6Val) that causes sickle cell disease when a person inherits two copies (homozygous). People with just one copy (heterozygous carriers) often resist severe Plasmodium falciparum malaria. That illustrates recessive disease and heterozygote advantage: why a harmful allele can stay common in a population.

Point mutationAutosomal recessiveBalancing selection

Inheritance:

Autosomal recessive disease: heterozygous carriers are often asymptomatic but carry one HbS allele that passes to roughly ~50% of their offspring through meiosis.

Gene function

HBB encodes β-globin, one of two protein chains that make up haemoglobin (HbA), the oxygen carrier in adult red blood cells. Two α-globin and two β-globin chains assemble around four iron-containing haem groups, one per subunit, and it is the haem iron that actually binds and releases oxygen. Haemoglobin works through cooperative binding: when one subunit binds oxygen, it shifts shape slightly, which makes the remaining subunits more eager to bind. This lets haemoglobin load up efficiently in the lungs and release oxygen precisely in tissues where it is scarce. Proper β-globin structure is essential to this mechanism; any change that disrupts the protein shape can compromise oxygen delivery throughout the body.

The sickle-cell mutation

The disease-causing change is a single nucleotide substitution in codon 6 of HBB: GAG (Glu) → GTG (Val). One DNA base changes from A to T, swapping glutamic acid, a charged, water-attracting amino acid, for valine, which is small and hydrophobic (water-repelling). The mutant protein is called HbS. In low-oxygen conditions, deoxy-HbS molecules polymerise; they stick together and stack into rigid fibres that distort the red blood cell into a sickle shape. In people with two copies of the mutation, this repeated sickling causes haemolysis (red-cell breakdown), vaso-occlusion (vessel blockage), and the chronic pain crises and organ damage that define sickle cell disease.

But how do we know?

Pauling et al. (1949) showed that HbS and HbA migrate differently during electrophoresis, proving the protein was structurally altered, the first molecular disease. Beet and Neel (1949) confirmed Mendelian recessive inheritance. Ingram (1956) used tryptic peptide fingerprinting to identify the Glu→Val swap at β6. DNA cloning and sequencing in the 1970s–80s then pinned the change to the precise GAG→GTG base change, each experimental step was independently testable and built on the one before.

Inheritance pattern

Sickle cell disease is autosomal recessive, a person needs two copies of HbS to develop symptoms. The most common form is HbSS (two sickle copies), but compound heterozygotes, people who inherit one HbS copy and a different, also non-functional, β-globin variant from the other parent, also develop clinical disease with variable severity. Heterozygous carriers (HbAS) produce both normal and sickle β-globin. They are usually symptom-free but gain significant protection against severe malaria caused by Plasmodium falciparum. This advantage explains why the sickle allele remains common in malaria-endemic regions even though two copies cause serious disease.

Why HBB is a landmark gene

Sickle cell disease earned the name "molecular disease" because it was the first condition traced from an altered protein all the way back to a single amino-acid change. Pauling (1949) showed the protein was different before the gene was even identified. Ingram (1956) pinpointed the exact swap. The gene's evolutionary history gave population geneticists one of their clearest cases of balancing selection: a harmful allele persists because carriers have a survival advantage. HbS frequency maps almost perfectly onto the historical range of Plasmodium falciparum, a near-perfect example connecting molecular genetics, evolutionary biology, and public health in a single story.

Key facts

Codon 6 GAG → GTG in HBB: a single A→T substitution swaps glutamic acid for valine in β-globin, producing HbS.
Autosomal recessive, homozygous HbSS (or compound heterozygotes with a second β-globin variant) is needed for clinical disease.
Heterozygous carriers (HbAS) are usually asymptomatic and gain significant protection against severe P. falciparum malaria.
Classic balancing selection, heterozygote advantage versus homozygote disease, explains high HbS frequencies in malaria-endemic regions.

Connect to the app

See the sickle-cell example in DNA & proteins, then trace how two recessive alleles can meet at fertilization in the meiosis explorer: linking translation, inheritance, and Punnett squares in one path.

HBB