How are cystic fibrosis and Tay-Sachs disease inherited?

Both cystic fibrosis and Tay-Sachs disease are inherited as autosomal recessive traits.123

In this pattern of inheritance, an individual must inherit two copies of the mutant gene—one from each parent—to manifest the disease. Parents who each carry one copy of the mutant gene and one normal gene are known as carriers; they are typically phenotypically unaffected because the normal, dominant allele masks the presence of the recessive mutation.423

When both parents are carriers, there is a one-in-four chance for each pregnancy that the child will inherit the mutant gene from both parents and be affected by the disorder. Because these conditions are recessive, they often appear "horizontally" in a family pedigree, meaning affected individuals are usually siblings within a single generation rather than appearing in every generation.42