Sickle -cell anaemia

Sickle-cell anaemia is caused by the defect in the production of haemoglobin. When a nitrogen base in the gene of haemoglobin is changed, the glutamic acid in position 6 of the normal beta chain would be replaced with valine. Then the abnormal haemoglobin is called the haemoglobin S, and sickle-cell anaemia is resulted.

The haemoglobin S has a smaller solubility at a lower partial pressure of oxygen. Then, it would crystallize out and change the shape of the red blood cell. These deformed cells would block the blood capillaries, causing impaired blood flow and tissue damage.

Homozygous patients have only haemoglobin S in their blood and can rarely survive to reproductive age.

Heterozygous patients have only about 30 to 40% haemoglobin S, and are said to show the sickle cell trait. Normally, their red blood cells are able to transport oxygen, but under conditions of oxygen shortage, the red blood cells collapse into sickle shape.

The distribution of the sickle cell allele is contained within the distribution of the malarial parasite, Plasmodium falciparum, and it has been shown that the sickle cell gene confers resistance to the diseasel, possibly because infected red blood cells become sickled and are removed by phagocytosis before the parasites reach maturity. In areas where malaria is endemic, heterozygous individuals are at an advantage compared with homozygous, SS, individuals, so that the sickle cell allele is maintained at a high level in spite of its disadvantages in the homozygous condition.

The symbols used are as follows :

Let	S be the dominant normal gene for the production of normal haemoglobin;
	s be the recessive gene producing haemoglobin S.

Then, the performance of various genotypes would be as follow :

SS	is the normal individual;
Ss	is the sickle cell trait carrier. He/she has 30 -40 % haemoglobin S in red blood cells.
ss	is the sickle cell anemia patient. He/she could hardly survive over puberty.

Reference :

(1)	(book)	Advanced level Biology for Hong Kong, Bk 2, by Y.K. Ho, Manhattan Press, pp. 201-202
(2)	(book)	Understanding Biology for Advanced Level, 4th Edition, by Glenn and Susan Toole, Stanley Thornes Press, pp. 189-190.
(3)	(book)	A-Level Biology, by W.D.Phillips and T.J.Chilton, Oxford University Press, pp.352-353.
(4)	(web)	Wikipedia
(5)	(web)	HealthNotes
(6)	(web)	Sickle cell society
(7)	(web)	MedicinePlus
(8)	(web)	MedicineNet

(03.08.2008)