What is the mode of inheritance for sickle cell anemia?

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"Sickle cell anemia is an autosomal recessive genetic disorder caused by a defect in the HBB gene, which codes for hemoglobin.  The presence of two defective genes is needed for sickle cell anemia."

What the above suggests is that both parents must possess the defective gene and these genes must be passed down to the offspring before the disease can occur.  "If each parent carries one sickle cell hemoglobin gene and one normal gene, each child has a 25% chance of inheriting two defective genes and having sickle cell anemia."

 The link below has more information on prevalence and symptoms.

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