The majority of human diseases, perhaps with the only exception of infectious diseases, has a genetic basis, or at least a genetic predisposition, even if this is often difficult to detect given that the clinical phenotype (ie, the manifestations of the disease) results from the interaction between individual genetics and the environmental factors (multifactorial diseases). The monogenic diseases are those conditions in which the alteration of a single gene is capable of causing the disease. To understand the basic rules of the transmission of genetic diseases, keeping in mind that our genetic heritage is double is necessary. In other words, we have two copies of every gene, one of which is inherited from the mother and another one is inherited from the father.
In arrhythmogenic genetic diseases the transmission can be:
Autosomal dominant: 50% chance that the disease is transmitted, regardless of the gender.
Autosomal recessive: the disease is clinically present only if the defect is inherited in double dose, ie by both parents. The genetic defect will be, therefore, present in homozygosity. The bearer of a single abnormal gene (heterozygous) can be defined as an healthy carrier of the disease. In a couple where both parents are carriers (heterozygous) you will have 25% chance of having a child who does not have the genetic defect, 25% chance of having a child homozygous (therefore manifesting the disease) and 50% chance to generate a heterozygote (healthy carrier). Autosomal recessive diseases occur more frequently when there is parental consanguinity.
In the reality of the clinical practice, two factors make it more complex to study the inheritance of a disease: the incomplete penetrance and the variable expressivity.
The penetrance is the ratio between the number of individuals who manifest the phenotype and the number of individuals affected; incomplete means that not necessarily all of the carriers of the genetic defect will show the disease. The variable expression is due to the fact that the same genetic defect may manifest differently in different individuals: this is why we have, in the same family, subjects with serious clinical manifestations and people with the disease in a mild form.