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Abnormal Karyotypes

Down’s Syndrome is  a common genetic abnormality referred to as trisomy 21. Instead of having the complement of 46 chromosomes of 22 homologous pairs plus 2 sex chromosomes, there are 47 chromosomes consisting of an additional chromosome 21.

Standard human karyotype with 46 chromosomes. Both XX and XY also shown here.

The appearance of extra or missing chromosomes arises during meiosis in an event called nondisjunction. After fertilization, a zygote with an improper chromosome complement occurs.

In nondisjunction, the chromatids do not properly separate during anaphase II of meiosis.
From left to right, the ploidy of the resultant zygotes: 2n, 2n, 2n+1, 2n-1; 2n+1, 2n+1, 2n-1, 2n-1

Nondisjunction can occur during meiosis I or meiosis II to yield the aneuploid states of 2n+1 or 2n-1.

Down’s syndrome karyotype showing the traditional trisomy 21

Translocation

Translocation is the movement of a piece or a whole chromosome onto another chromosome. This more often involves chromosomes that are acrocentric. An acrocentric chromosome has its centromere closer to the end of the chromosome instead of near the middle. The acrocentric nature of chromosome 21 and its small size makes it prone to an event called Robertsonian translocation whereby two acrocentric chromosomes fuse. This results in translocation Down’s syndrome, which has the same effects as inheriting an extra copy of chromosome 21.

Down’s Syndrome from a 14:21 translocation.

Abnormalities in mitosis also occur and can result in diseases from translocations.

Brain Cancer Chromosomes
A spectral karyogram of a brain cancer (glioblastoma) illustrating multiple translocation problems and chromosome instability
Diseases and conditions caused by translocations.
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