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Euploidy and aneuploidy describe having a number of chromosomes that is an exact multiple of the number of chromosomes in a normal gamete; and having any other number, respectively. For example, a person with Turner syndrome may be missing one sex chromosome (X or Y), resulting in a (45,X) karyotype instead of the usual (46,XX) or (46,XY).
[5] [6] Aneuploidy can be full, involving a whole chromosome missing or added, or partial, where only part of a chromosome is missing or added. [5] Aneuploidy can occur with sex chromosomes or autosomes. [citation needed] Rather than having monosomy, or only one copy, the majority of aneuploid people have trisomy, or three copies of one chromosome.
The unequal distribution of DNA to daughter cells upon mitosis results in a failure to maintain euploidy (the correct number of chromosomes) leading to aneuploidy (incorrect number of chromosomes). In other words, the daughter cells do not have the same number of chromosomes as the cell they originated from.
Aneuploidy is the presence of an abnormal number of chromosomes in a cell, for example a human somatic cell having 45 or 47 chromosomes instead of the usual 46. [1] [2] It does not include a difference of one or more complete sets of chromosomes. A cell with any number of complete chromosome sets is called a euploid cell. [1]
Differences in degree and distribution of GC content (Guanine-Cytosine pairs versus Adenine-Thymine). In metaphase where the karyotype is typically studied, all DNA is condensed, but most of the time, DNA with a high GC content is usually less condensed, that is, it tends to appear as euchromatin rather than heterochromatin.
Although the replication and transcription of DNA is highly standardized in eukaryotes, the same cannot be said for their karyotypes, which are highly variable between species in chromosome number and in detailed organization despite being constructed out of the same macromolecules. In some cases, there is even significant variation within species.
The insecticides fenvalerate and carbaryl have been reported to increase spermatozoa aneuploidy. Occupational exposure of pesticide factory workers to fenvalerate is associated with increased spermatozoa DNA damage. [7] Exposure to fenvalerate raised sex chromosome disomy 1.9-fold and disomy of chromosome 18 by 2.6-fold (Xia et al., 2004).
Aneuploidy of autosomes is not well tolerated and usually results in miscarriage of the developing fetus. Fetuses with aneuploidy of gene-rich chromosomes—such as chromosome 1 —never survive to term, [ 9 ] and fetuses with aneuploidy of gene-poor chromosomes—such as chromosome 21 — are still miscarried over 23% of the time. [ 10 ]