Inherited disorder
A condition caused by faulty alleles passed from parents to children.
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Analogy
A typo in the recipe copied into every new book.
In practice
Cystic fibrosis is caused by a recessive allele. Huntington's disease is caused by a dominant one.
Questions people ask
Can inherited disorders be screened for?
Yes. Embryos made by IVF can be tested before being placed in the womb, and adults can have genetic tests to see if they carry a faulty allele. Screening raises ethical questions, such as who should be tested and how the results should be used, which people debate.
What is cystic fibrosis?
Cystic fibrosis is an inherited disorder caused by a recessive allele. It makes the body produce thick sticky mucus in the lungs and gut, which causes breathing and digestion problems. A child gets the disease only if both parents are carriers and each passes on the faulty allele, a 1 in 4 chance.