Umair,
Yes, you are right. the "weak" genes in thalassemia are the mutated or deleted genes and it is more likely that relatives will carry these genes. This is especially true in 'closed" cultures such as island cultures where most people do have at least some distant relation to each other. I myself, am the "victim" of this. My English grandparents were cousins (and back in those days this was not uncommon) and they both carried a bad gene that leads to thyroid disorders. I have a severely under active thyroid. My mother, three of my brothers, my sister, my daughter, my niece and myself are all confirmed cases of thyroid disorder. This is all because relatives who had the same faulty gene, married and had children, and passed both of these genes on to my mother. My mother most likely had two bad genes and when combined with my father's gene (he was diagnosed with thyroid disorder later in life) it resulted in a family where thyroid disorders are prevalent.
The lesson, and especially with blood disorders which can be fatal, is that it is extremely important to know your family history and to pass this on to your own children, so they too are aware that they may also carry a bad gene and can take precautions to prevent their children from having the major disorders and also educating their children so they know enough to also take precautions. Yet, we still see, as with our new member, Cindy, that a doctor doesn't want to run a test that could verify or rule out if she is a thalassemia carrier. There are 2 million thal carriers just in the US, yet thalassemia is almost unknown here. Uncooperative doctors will not help matters one bit. There is a rapidly increasing rate of thal major in the US and the only way this can be stopped is by testing, education and awareness, and perhaps it is the medical establishment that needs the most educating.