!-Thalassemia and "-thalassemia

The thalassemias have a wide spectrum of
different genotypes and phenotypes (disease
manifestations and course). In the "-thalassemias
(1), complete absence ("0) is distinguished
from decreased formation ("+) of the "
chain. With the !-thalassemias (2), one, two,
three, or all four loci for !-globin may be affected.
Altogether, there are 12 principal genotypes.
In individuals with two mutations at the
!-loci (!-thalassemia), the two can lie either on
the same chromosome (thal-1) or on different
chromosomes (thal-2). Thal-1 occurs mainly in
Southeast Asia; thal-2, mainly in Africa. Each !
gene is located within a 4 kb region of homology,
interrupted by small, nonhomologous
regions. The most frequent mechanism for the
origin of a chromosome with only one !-globin
gene is nonhomologous crossing-over between
two !-globin gene loci after mispairing of the
homologous chromosomes during meiosis.