Genetics, so what you inherit from your father and give to your children, only accounts for some of the risk of getting coeliac disease. So even in monozygotic twins that have identical DNA, one may develop coeliac disease and the other not. Your family members have far less shared DNA, than in the case of identical twins, so the chances of getting coeliac disease in family members may differ based on their genetic background. Moreover, other factors that are not in the DNA are also thought to play a big role in getting the disease.

These factors are not very well known yet, but we think that some virus infections or the gut microbiota may play a role. We need to learn a lot more about these factors from our DNA and our surroundings to be able to understand why one family member gets coeliac disease and the other does not, but we are working on it!