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In an autosomal recessive disorder, the abnormal (mutated) recessive gene is located on one of the nonsex chromosomes (autosomes). To inherit an autosomal recessive disorder — such as cystic fibrosis, sickle cell anemia or phenylketonuria (PKU) — both parents must be carriers. The child inherits two copies of the abnormal gene — one from each parent.
A carrier is an unaffected individual who has one copy of the abnormal gene and the other copy is normal. With each pregnancy, two carriers have a 25% chance of having an unaffected child with two normal copies of a gene (left), a 50% chance of having an unaffected child who also is a carrier (middle), and a 25% chance of having an affected child with two abnormal copies of the gene (right).
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