Explain sickle cell Anaemia.

Explanation

This is an inborn error where the normal biconcave erythrocytes of an individual are transformed into crescent or sickle shape. This is due to the production of defective Haemoglobin. The production of normal Hb is controlled by the gene Hb whereas the other allele Hb controls the production of s-haemoglobin or sickle cell haemoglobin. Therefore, this disease is not due to recessive genes but due to partially dominant pairs of alleles. When Hb occurs in a homozygous condition` (Hb^(A) Hb^(A))` normal hemoglobin is produced in the body and the person will be healthy. But the presence of Hb- in homozygous `(Hb^(3) Hb^(3))` condition makes the person suffer from chronic haemolytic anaemia. It is due to the production of Hemoglobin, which contains the amino acid valine instead of glutamic acid. This defective Hemoglobin cannot transport oxygen. As a result, the RBC are deformed into sickle shape. Such RBCs are not able to move through the blood capillaries and therefore cause internal bleeding and pain. Such patients become anaemic and die. The genetic code and the codon for glutamic acid is as follows: DNA code :   CTT
III
MRNA codon: GAA
Results: Glutamic acid is in the 6th position.
But under abnormal condition, instead of glutamic acid at the 6th position, valine, whose code and codon is as follows
: DNA code CTT 
III 
MRNA codon GUA
Result: Vuline in the 6th position
  In heterozygous conditions, both Hb and Hbr genes are present. Such individuals produce both normal and S-haemoglobin This indicates that the genes HbA and Hbs are co-dominant such individuals s er from periodi d o rm emis at ligler altitudes