Question

Does a fetus homozygous for sickle-cell hemoglobin (Hb S) have normal Hb F?

Short answer

Yes, a fetus homozygous for Hb S can have normal Hb F.

Step-by-step solution

- Understanding the genetic makeup

Determine the genetic makeup of a fetus homozygous for sickle-cell hemoglobin (Hb S). A fetus with this condition has two copies of the Hb S gene (one from each parent), making its genotype Hb SS.

- Understanding hemoglobin types

Hemoglobin (Hb) types vary during different stages of life. Fetal hemoglobin (Hb F) is the predominant type in fetuses and newborns. Hb F usually transitions to adult hemoglobin (Hb A) and other variants after birth.

- Exploring the presence of Hb S and Hb F

Hb S is a variant of the adult hemoglobin that causes sickle-cell disease. However, fetal hemoglobin (Hb F) production is not influenced by the sickle-cell trait (Hb S). Therefore, a fetus with Hb SS can still produce normal levels of Hb F.

- Conclusion

Given the above points, a fetus homozygous for sickle-cell hemoglobin (Hb S) can still have normal Hb F because Hb F production is independent of the Hb S genotype.

Key concepts

fetal hemoglobin (Hb F)

Fetal hemoglobin, or Hb F, is the primary type of hemoglobin found in fetuses and newborns. Hemoglobin is the molecule in red blood cells responsible for carrying oxygen from the lungs to the body's tissues.

Hb F differs from the adult form of hemoglobin, Hb A, in its structure and oxygen-binding properties. This difference allows Hb F to effectively transport oxygen across the placenta from the mother to the fetus.

After birth, infants gradually replace Hb F with Hb A. But during fetal development, Hb F is crucial for providing sufficient oxygen to the growing baby.

sickle-cell disease

Sickle-cell disease (SCD) is a genetic condition that affects the shape and function of red blood cells. Normally, red blood cells are round and flexible, allowing them to move easily through blood vessels.

In individuals with SCD, the presence of sickle-cell hemoglobin (Hb S) causes red blood cells to become rigid and crescent-shaped, resembling a sickle.

These abnormally shaped cells can block blood flow, leading to painful episodes, organ damage, and increased risk of infections.

• SCD is inherited in an autosomal recessive pattern.
• Individuals with two copies of the Hb S gene (Hb SS) have sickle-cell disease.
• Those with one Hb S and one normal Hb gene (Hb AS) are carriers, typically showing no symptoms themselves.

Despite the issues caused by Hb S, it does not affect the production of fetal hemoglobin (Hb F), allowing fetuses with SCD to have normal oxygen levels.

genetic makeup of hemoglobin

Hemoglobin genetic makeup determines the type and functioning of hemoglobin in the body. Genes inherited from parents encode the production of hemoglobin molecules.

For hemoglobin, several different genes contribute to its structure. These include the beta-globin gene for adult hemoglobin (Hb A) and gamma-globin genes that produce fetal hemoglobin (Hb F).

• A normal genotype includes two Hb A alleles (Hb AA) for adult hemoglobin.
• Sickle-cell disease arises from inheriting two Hb S alleles (Hb SS), where the beta-globin gene is mutated.

Despite these mutations, the production of fetal hemoglobin (Hb F) is not affected by the sickle-cell trait. Consequently, fetuses with the Hb SS genotype can still produce normal Hb F levels, essential for their development.