Understanding HBA1, HBA2 Genes and Blood Disorders

The question asks which condition is associated with the two closely linked genes, HBA1 and HBA2. These genes play a crucial role in producing components of haemoglobin, the protein in red blood cells that carries oxygen.

What are HBA1 and HBA2 Genes?

The HBA1 and HBA2 genes are located on chromosome 16. They provide instructions for making a protein component called the alpha-globin chain. Haemoglobin in adults is primarily made up of two alpha-globin chains and two beta-globin chains.

Association with Alpha Thalassemia

Alpha thalassemia is a blood disorder characterized by reduced production of alpha-globin chains. This reduced production is most commonly caused by deletions or mutations in the HBA1 and HBA2 genes. Since humans typically have two copies of the HBA1 gene and two copies of the HBA2 gene (a total of four alpha-globin genes), the severity of alpha thalassemia depends on how many of these genes are affected.

• Loss of one alpha-globin gene copy usually causes no symptoms.
• Loss of two alpha-globin gene copies leads to alpha thalassemia minor, a mild form.
• Loss of three alpha-globin gene copies results in Hb H disease, a moderately severe anaemia.
• Loss of all four alpha-globin gene copies causes hydrops fetalis, a severe condition often leading to stillbirth or death shortly after birth.

Therefore, the HBA1 and HBA2 genes are directly associated with alpha thalassemia.

Why Other Options Are Incorrect

Let's consider why the other options are not associated with the HBA1 and HBA2 genes:

• β thalassemia: This is a disorder characterized by reduced production of beta-globin chains. It is caused by mutations in the HBB gene, which is located on chromosome 11, not chromosome 16.
• Haemophilia: This is a bleeding disorder caused by deficiencies in blood clotting factors. The most common types, Haemophilia A and B, are caused by mutations in genes for Factor VIII (F8 gene) and Factor IX (F9 gene), respectively, both located on the X chromosome. These are unrelated to globin genes.
• Sickle cell anaemia: This is a genetic disorder that affects beta-globin, causing red blood cells to become sickle-shaped. It is caused by a specific mutation in the HBB gene on chromosome 11, not the HBA1 or HBA2 genes.

Conclusion

Based on the genetic basis of these conditions, the HBA1 and HBA2 genes are specifically linked to the production of alpha-globin chains, and defects in these genes cause alpha thalassemia.

Revision Table: Blood Disorder Genetics

Additional Information: Globin Genes and Haemoglobin

Haemoglobin is a complex protein made of four globin chains (usually two alpha-like and two beta-like) and an iron-containing heme group. Different globin genes are expressed at different stages of development:

• Embryonic Haemoglobin: Contains zeta (ζ) and epsilon (ε) globin chains. Genes are HBE1 and HBZ.
• Fetal Haemoglobin (HbF): Contains alpha (α) and gamma (γ) globin chains. Genes are HBA1, HBA2, and HBG1, HBG2.
• Adult Haemoglobin (HbA): Contains alpha (α) and beta (β) globin chains. Genes are HBA1, HBA2, and HBB.
• Adult Haemoglobin A2 (HbA2): Contains alpha (α) and delta (δ) globin chains. Genes are HBA1, HBA2, and HBD.

The HBA1 and HBA2 genes are part of the alpha-globin gene cluster on chromosome 16. The beta-globin gene cluster, located on chromosome 11, includes HBB, HBD, HBG1, HBG2, and HBE1. Disorders affecting these gene clusters are collectively known as haemoglobinopathies.