A hereditary disease carried by a single recessive gene. When the gene is in homozygous (paired) condition in an individual, the disease is expressed. It causes the red blood cells to collapse and stack into columns under acidic blood conditions, as happens for example with physical exertion. The "rods" of stacked blood cells can clog blood vessels and limit the effective flow of oxygenated blood around the body. The shape of the "sickled" blood cells (as seen under a microscope) triggers a response in the body which very quickly destroys them, so adding to the symptoms of anaemia.
It is believed that sickle cell anaemia is the genetic "trade-off" for a beneficial gene mutation which protects against malaria, and so saves more lives than it kills. The waste products of the malarial parasite are acid, and so infected blood cells capable of "sickling" trigger the body to destroy them before the parasite can multiply and invade other cells. For carriers of a single sickle cell gene, only a small proportion of the red blood cells have the ability to "sickle", but this number is sufficient to afford protection against malaria without weakening general health.