2.6 Flashcards
Haemoglobin is a protein found in red blood cells that
Carries oxygen
Sickle-cell anaemia is an inherited condition in which affected individuals produce an
Abnormal type of haemoglobin called haemoglobin S
Sickle-cell haemoglobin has a very small difference in the
Order of amino acids in two of the polypeptide chains that make up each molecule
As a result of this difference in order of amino acids in two of the polypeptide chains, haemoglobin S is
Less soluble than normal
Crystallises in low oxygen concentrations and become sickle shaped when oxygen is low
Sickle shaped cells have difficulty in passing through small blood vessels, so they can block capillaries which means that
Less oxygenated blood reaches affected parts of the body and cells die
Causes damage to tissues and severe pain in the affected individual
Sickle cells may block the flow of blood through capillaries in the lungs, causing
- Lung tissue damage (acute chest syndrome)
- Severe pain in the arms, legs, chest and abdomen
- Strokes when parts of the brain tissue die
- Damage to most organs (kidneys and liver)
Children with sickle-cell anaemia often suffer damage to the spleen early in life, which leaves them
Vulnerable to certain bacterial infections
The sickle-shaped cells are easily damaged and are
Rapidly broken down causing anaemia
Anaemia means that the affected person has
Fewer red blood cells
Lower concentration of oxygen in their blood
People with sickle-cell anaemia are treated with:
Blood transfusions
Intravenous fluids
Antibiotics
Painkillers
However without treatment the condition is usually fatal in
Childhood
Sickle-cell anaemia is determined by a single gene with
Two alleles
Sickle cell anaemia alleles are not dominant or recessive they are
Codominant
Codominant means that
Both alleles contribute to the phenotype in a heterozygous individual
Because these Sickle cell anaemia alleles are co dominant the symbol for normal haemoglobin is
HbA
Allele for sickle cell haemoglobin is
HbS
A person with the genotype HbAHbS is heterozygous and produces
Both kinds of haemoglobin in their red blood cells
HbAHbs also means that although they produce both kinds they do not produce enough sickle-cell haemoglobin to show symptoms of sickle-cell anaemia unless they are in conditions where oxygen is limited. These people are said to have
Sickle-cell trait
Sickle cell anaemia is very rare in Northern Europeans, but much more common among people in Africa or of
African origin
Sickle cell allele is particularly common in parts of Africa and India. These are also the areas where the disease
Malaria is most common
In these areas where malaria is most common, people with normal haemoglobin (HbAHbA) do not suffer from Sickle cell anaemia but are more likely to die from malaria. Those with sickle cell trait (HbAHbS) have some protection from malaria so are
More likely to survive, reproduce and pass on their alleles
Because natural selection favour heterozygotes in areas where malaria is common, it is inevitable that children with Sickle cell anaemia will be
Born
