3.2 Applications and skills
3.2.1 Sickle cell anemia
- Red blood cells contain globular proteins called hemoglobin, which are made of two α- and two β- protein subunits.
- Sickle cell anemia is a blood disorder caused by a single base substitution in the gene that codes for the β- subunit of hemoglobin.
- When the mutation is transcribed by mRNA, the resulting codon also contains a substitution and when the mRNA is translated, the mutant polypeptide has valine instead of glutamic acid as the sixth amino acid in the polypeptide chain.
- Valine and glutamic acid have different chemical properties, so the polypeptide folds differently, the hemoglobin molecule differs in configuration and the red blood cell develops into a sickle shape.
- Sickle-shaped red blood cells deliver oxygen inefficiently and their irregular shapes often block blood vessels. A person with the disease may develop severe symptoms of anemia. The disease is deadly when left untreated.
- Sickle cell anemia is inherited in an autosomal recessive pattern. This means that both copies of the gene must show the sickle cell trait in order to show symptoms of the disease.
- Carriers who are heterozygous for the sickle cell trait do not show symptoms of the anemia but do have sickle-shaped red blood cells.
- In other words, the sickle cell allele is co-dominant with the normal hemoglobin allele since both alleles affect the phenotype (shape of red blood cells).
and the sickle cell trait
- Sickle cell anemia is often deadly, yet approximately 40% of people of African descent are carriers of the trait.
- The geographical distribution of the sickle cell allele is very well correlated with the incidence of malaria. Malaria is caused by a parasite that is transmitted to the bloodstream by mosquitoes.
- Numerous studies have shown that carriers of the sickle cell trait have lower counts of the malaria-causing parasite in their bloodstream.
- Scientists have long suspected that the sickle cell trait somehow protects infections by the parasite that causes malaria.
- This hypothesis is known as the ‘heterozygous advantage’. Despite its deadly effects, the sickle cell trait has been maintained in the population because carriers are protected from the harmful effects of another disease.
Figure 3.2.1h – Mosquito
Mosquitoes transmit the parasite Plasmodium, which causes malaria
Humans have the vast majority of base sequences in common, but there are many single nucleotide polymorphisms that contribute to human diversity.