Why can cystic fibrosis appear in a family with no previous history of it?

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Question

Why can cystic fibrosis appear in a family with no previous history of it?

Accepted Answer

The recessive allele can be carried unnoticed through several generations — Nothing shows until two carriers have a child together.

Answer

It is caused by the environment rather than alleles

Answer

The allele appears spontaneously in the affected child only

Answer

The recessive allele can be carried unnoticed through several generations

Answer

It can be caught from another family member

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