In most families, long QT syndrome follows autosomal dominant inheritance. This means a single changed copy of the gene is enough to cause the condition. We all carry two copies of each gene — one from each parent. A person with long QT syndrome has one working copy and one changed copy.

Each time that person has a child, there is a 1-in-2 chance of passing on the changed copy, and a 1-in-2 chance of passing on the working copy. This chance is the same for every pregnancy, and for sons and daughters alike. Almost everyone with long QT syndrome has inherited it from a parent; new variants that start in the affected person are uncommon.

An important feature is that the same variant can affect family members very differently. This is called variable expression and reduced penetrance. About 1-in-4 people who carry a disease-causing variant have a normal electrocardiogram. They may never have symptoms, but they still carry some risk — especially with certain medicines or low potassium — and can still pass the variant to their children. This is why a normal electrocardiogram alone cannot rule out long QT syndrome in a relative.

A rare recessive form: Jervell and Lange-Nielsen syndrome When a child inherits a changed copy of KCNQ1 (or, less often, KCNE1) from both parents, they have Jervell and Lange-Nielsen syndrome. These children are born with profound hearing loss and a much longer QT interval, and have a higher risk of heart events. When both parents carry a variant, each pregnancy has a 1-in-4 chance of this form. It is more common in communities where marriage between relatives is traditional, which is relevant to some families in India.

Once the family’s variant is known, “cascade testing” is recommended for parents, brothers, sisters, and children:
Genetic testing for the known family variant A simple, accurate blood or saliva test that tells relatives whether they carry the variant.
Electrocardiogram and heart review Recommended for all first-degree relatives, whether or not they have genetic testing.
Testing children from birth Children can be tested from birth, because heart events can happen in childhood and treatment is effective.

Testing during pregnancy or before pregnancy (for example, with embryo testing during in vitro fertilisation) is possible when the family variant is known, and is best discussed with a genetic counselor.