It's a autosomal recessive mutation in the HBB gene. This gene codes for hemoglobin.

No one knows precisely how it originally mutated. But it is now passed from parent to child. Because it is recessive, it is possible to carry the defective gene without suffering symptoms. Thus if two parents each are carriers, their child will have a 25% chance of having the disease, a 50% chance of being a carrier, and a 25% chance of having two normal copies of the gene.
Source(s):
http://www.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/sca.s...

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