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Sickle Cell Disease and Trait
Sickle Cell Disease:
Sickle Cell Disease (SCD) is a condition that is determined by the inheritance of a pair of alleles,
which are recessive to the allele for normal shaped red blood cells. If a child inherits two alleles for
Sickle Cell, his red blood cells take on a sickle shape. The sickle shape causes problems by not
easily passing through narrow blood capillaries. Sickle cells are destroyed in the spleen, reducing
the number of red blood cells in the body and thus decreasing the amount of oxygen available to
cells in
the body.
Sickle Cell Trait:
If a child inherits only one allele for sickle cell, he has the Sickle Cell Trait (SCT), a heterozygous
genotype, but does not have the disease. Most people with Sickle Cell Trait have no symptoms at all
and a rare few may show complications due to decreased oxygen in the blood. Increased
atmospheric pressure when diving, extreme exercising, dehydration or being in high altitudes may
bring on complications. These include pain and swelling when the sickled red blood cells get stuck
in blood vessels and may result in heat stroke and muscle breakdown.
Natural Selection:
1 in 12 African Americans in the U. S. have one allele for sickle cell or the Sickle Cell Trait. In
Africa there is a higher frequency of the sickle cell gene. Heterozygous individuals, who have the
Sickle Cell Trait, are resistant to malaria, a leading cause of death in Africa. Having the Sickle Cell
Trait is an advantage for survival and these individuals have a greater chance of passing their genes
on to the next generation.
Problems:
1. If we let s represent the sickle cell allele and S represent the normal red blood cell, what would be
the probability of two heterozygous parents producing a child with Sickle Cell Disease? What is the
probability of carriers (heterozygous or SCT)? Probability of individuals with normal red blood
cells? Show the Punnett Square with your answer.
2. What would be the probability of a carrier (heterozygous) parent who marries a person with a
homozygous normal genotype for the shape of red blood cells producing a child with Sickle Cell
Disease? Show the Punnett Square with your answer.