Autosomal Trisomies
A trisomy (the presence of three homologous
chromosomes instead of the usual two) arises
prezygotically during meiosis due to faulty distribution
(nondisjunction) of a chromosome
pair. Itmay also arise after fertilization (postzygotic)
during somatic cell division (mitosis); in
this case, trisomy is present in a certain proportion
of cells (chromosomal mosaicism). Trisomy
leads to a phenotype characteristic for the particular
chromosome, although in humans most
trisomies are lethal in early embryonic development.
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