In order for the disorder to be expressed, each parent must be a carrier of an mutated copy of the same gene, and each must pass on that copy to his/her offspring. Each child born to parents who are carriers has a 25% chance of having the disorder (by inheriting the mutated copy from both parents), a 50% chance of being a carrier (by inheriting an mutated copy from one parent and a healthy copy from the other) and a 25% chance of inheriting a healthy copy of the gene from each parent.

(Also called Recessive Inheritance) A pattern of inheritance in which a gene change has an effect only if a copy is inherited from both parents, e.g. the mutated gene that causes cystic fibrosis.

A genetic disorder that appears only in patients who have received two copies of a mutant gene, one from each parent.

An inherited problem which occurs only when a particular inherited gene is inherited from both parents.

a gene on one of the first 22 pairs of chromosomes, which, when present in two copies, causes a trait or disease to be expressed.