The inherited disorder of haemoglobin (Haemoglobinopathies) are so common in the carrier form and so lethal in the disease form that an explanation for their extraordinarily high frequency is required.

The incidence of thalassaemia trait varies greatly from country to country. It's extremely widespread and occurs in a line extending through the Mediterranean, the Middle East, the Indian sub-continent and through out South East Asia, in a region including Southern China, Thailand, the Malay Peninsula and many of the islands.

Because the world distribution of the major inherited haemoglobin disorders coincides with that of malaria, it's thought that carriers of the thalassaemia gene have some protection from the most severe forms of malaria.