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Mutations - Aneuploidy

Aneuploidy describes the case in which one to a few whole chromosomes is added to or lost from the normal set of chromosomes in a cell. Although aneuploidy can occur by several mechanisms, the most important is nondisjunction, as we discussed in the previous section. Since aneuploidy is often lethal in animals, karyotype analysis is frequently used to try to diagnose aneuploids early in development.

Karyotypes are pictorial representations of all of the chromosomes in an organism. They are constructed by taking a picture of the metaphase chromosomes from a single nucleus. Next, the homologous chromosomes are paired up, and the pairs are arranged in groups of similar morphology with roughly decreasing size. Geneticists use karyotypes to determine if individuals have the correct number of chromosomes and to look for any large chromosomal mutations.

The karyotype of a human aneuploidy, Down Syndrome, is shown in the figure to the right. Note that the individual shown here has three copies of chromosome 21, instead of the normal two copies displayed by the rest of the chromosomes. Thus, Down Syndrome is also referred to as "trisomy 21." Individuals who suffer from Down syndrome exhibit such abnormalities as low IQ, short and broad hands, and below average height.