Hello, there are more than one sort of thalassemia. Since your boy s got 3 years old before anybody noticed, he most likely has minor thalassemia which is usually a pretty mild condition that only means a somewhat lower hematocrit than normal. If this is the case with him then this means that one of the parents (or both) is carrying a defective gene. Thalassemia major causes characteristic appearance changes that should get noticed by doctors. Patients with Thalassemia major require transfusions every so often. Let me describe to you how these genes get from the parents to their kids. Each parent has two genes. Each kid inherits two genes, one from each parent. This means that two people with thalassemia minor (one defective gene each) may have a severely sick child with 2 defective genes (thalassemia major). If the kid has one defective gene then this is thalassemia minor. This procedure is a rather simple one although it may sound confusing at first. So the answer is that your other kids may have the defective gene as well and they should be tested. I hope I ve been clear enough. If you need further clarifications, please ask and I ll be glad to help. Kind Regards!