According to the different location of the mutations we have different types of thalassaemias.

The most common are:

• β thalassaemias characterized by the increased Hb A₂
• α thalassaemias that shows a normal haemoglobin picture
• F thalassaemias in which the Fetal Hb increases
• thalassaemia with Hb Lepore
• thalassaemia with Hb E

Within the β thalassaemias varieties are distinguished in which the molecular defect causes the complete absence of the chain synthesis (β⁰thal) and varieties in which the defect only cause a reduction of the β globin chain synthesis (β⁺ thal).

Molecular variety of β thalassaemias most frequent in Italy

Despite the high number of evidenced thalassaemic defects, in every area, only few defects are always predominant.

In Italy the most frequent varieties of β thalassaemias are:

• β⁰ mutation, codon 39, C→T;0;
• β⁺ mutation, IVS -110, G→T;
• β⁺ mutation, IVS -6, T→C;
• β⁺ mutation, IVS II-745, C→G; + ;
• β⁺⁺ due to -101, C→T, -87 C→T, -86 C→A mutations of the β gene promoter;
• Sicilian type deletion of the δ and β genes;
• partial deletion of the δ and β genes that gives rise to the Hb Lepore;
• α locus triplication of a chromosome.

The globin genes' molecular studies have definitely shown that the thalassaemias, due to the β⁰39, IVS I-110 and IVS II-745 mutations are manifest with a marked thalassaemic haematological picture, while the thalassaemias due to the β IVS I-6, β -87 and β -86 mutations have a lighter picture. The thalassaemias, due to the β -101 mutation or to the α locus triplication of a chromosome 16, very often have a completely normal haematological picture.

Molecular variety of α Thalassaemias most frequent in Italy

The α⁺ thalassaemias are very often caused by deletion (the most frequent is the 3.7 Kb deletion, which only leaves in the cluster the - α^3.7 hybrid gene) and more rarely by point mutation. The α thalassaemias due to this type of deletion very often show a completely normal haematological picture as the haemoglobin one. These two characteristics of normality notably complicate the α thalassaemia diagnosis.

The α⁰ thalassaemias, very less frequent than the α⁺ ones, are caused by deletions involving both the α genes on chromosome 16, of which the - - MED and - α^20.5 deletions are the most frequent in Italy. Moreover, they are also caused by deletions or mutations involving an α gene located on each of the two chromosomes 16 (i. e. - α^3.7/ - α^3.7). The α⁰ thalassaemias reveal evident alterations of the haematological picture and normal haemoglobin one.

Variety of β thalassaemias most frequent in the world

The β⁰ 39 mutation, the most frequent in Italy, is very spread in the north area of the basin of the Mediterranean one (Spain, Portugal, southern France) while is becoming less diffused in Greece, Turkey, Libya and is being absent in Egypt as in all the zones of the southeast Asian.

The thalassaemia with HbE is very frequent in Thailand, Laos and Cambogia.

In Brazil the mutations are the same of the Mediterranean zone with an elevated frequency of the HbS coming from Africa.

Variety of α thalassaemias most frequent in the world

The frequency of the α thalassaemia is very higher than the β one, in all the territories at elevated incidence. In a vast Asian territory that includes Thailand, Laos, Burma, southern China, a pathology is diffused, incompatible with the life: the hydrops ascites foetal. This genetic alteration is the homozygous condition for a α⁰ thalassaemia. It causes frequent premature births with a brief survival of the newborn within few days.