A-T is what is called an ‘autosomal recessive’ disorder. What this means, in simple terms, is that to be affected by the condition a person has to have two copies of a particular (mutated) gene.
For this to happen, both parents have to be carriers of the ATM gene. Almost always both parents will have one normal copy and one copy with a mutation. Some people with A-T do have children and in those cases they as the parent will have two mutated copies.
Each parent passes on (at random) one or other copy of the gene. As the diagram shows, there are four possible outcomes: the child has A-T (one in four chance), the child is a carrier (two in four chance) and the child does not have the mutation at all (one in four chance).
On average, one in four children born to a couple of carriers will have A-T. However the chance doesn’t change if you already have a child with A-T. Even if you already have one (or more) children with A-T, the chance of the next child having A-T is still one in four. There are a number of families in the UK and Ireland with two and even three children with A-T.
Siblings of a child with A-T, who don’t have A-T themselves, will each have a two out of three chance of being a carrier.