Answer :
Hemoglobin is the molecule that transfers oxygen through the cardiovascular system. In most mammals, hemoglobin consists of 4 distinct subunits. These subunits consist of an aminoacid chain (aminoacids are the building blocks of proteins) and a heme group (heme groups are ringlike molecules with an iron ion in the middle). 2 of the subunits have a protein chain with a-structure and 2 have a protein chain with beta structure. There are alleles that are responsible for the production of both kinds of chains and if there is a mutation in one, hemoglobin chains of that kind will have a different aminoacid order and thus they will fold differently in space. Thus, they will not be able to form the compound hemoglobin molecule (the 4 subunits won't "fit"), leading to symptoms of thalassemia. Stated shortly, beta chains are a protein chain that is critical to the hemoglobin molecule because every molecule needs 2 such chains that are foldable in a specific way. If this does not happen, the beta chains cannot be utilized for hemoglobin molecules and thus the organism will have trouble with oxygen transfer in its tissues.