This is a genetic disease in which abnormal haemoglobin known as haemoglobin S of the red blood cell crystallize under low oxygen concentration of the blood resulting in great pains. A red blood cell normally has its shape as a biconcave disc, but the red blood cells of some people have a sickle shape. This shape is caused by abnormalities in the haemoglobin. The shape of the red blood cell is controlled by an allele called H. this is responsible for the formation of normal haemoglobin and hence produces normal red blood cells. H is dominant allele and its corresponding recessive allele is S. this allele is responsible for the formation of abnormal red blood cells, called sickle cells. People who have sickle cell in their blood suffer from anaemia because the abnormal haemoglobin is not as efficient at carrying oxygen as the normal pigment. This condition is called sickle cell anaemia. People suffering from sickle-cell anaemia are not resistant to malaria and are often sick, weak and thin. Other symptoms are kidney and heart failure. Sufferers are referred to as sicklers. Sickle-cell anaemia is an inherited disease. People who are homozygous for the dominant allele, HH, have only normal red blood cells in their body. Those who are heterozygous (HS) have about 60% - 70% of normal red blood cells and 30% - 40% sickle cells in their body and appear phenotypically normal. These people have a high resistance to malaria as the parasite is unable to invade the sickle-shaped cells. This is because they allele H is not completely dominant over the allele S, which expresses its characteristic slightly in the heterozygote. A person heterozygous for the gene is said to have sickle-cell trait. Homozygous recessive people (SS) have little chance of survival without medical care.
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