Beta chain synthesis is controlled by two gene loci; one on each of chromosome 11. Chromosome 11 also carries the gene loci for delta chains, G-gamma and A-gamma chains and embryonic epsilon chains. Normal chromosome 11 is depicted in the image below.
In the genotypic notation of beta thalassemia, recall that a "+" represents a reduction in beta chain production whereas a "0" represents a complete deletion of a locus. The "+s" or "sc" represents a silent carrier. Delta chain deletions may be present in combination with beta chain deletions.(drawing modified from Harmening, 1999)