The Basics...

What is a karyotype?

A karyotype is when scientists take the chromosomes from the nucleus of a cell, pair them with their homologous chromosome, and line them up from longest to shortest. This process allows them to easily count the number of chromosomes and look for any extra or missing chromosomes. For example, humans should have 23 pairs of chromosomes. The first 22 pair are called autosomes and the last pair are the sex chromosomes (XX or XY). A karyotype may show an extra chromosome number 21 (three copies instead of two). This would be known as trisomy 21, or Down's Syndrome. There are, of course, other conditions with an abnormal chromosome count. The generic term for an abnormal chromosome count (more or fewer) is aneuploidy. When you have an extra chromosome, it may also be referred to as polyploidy. Most types of aneuploidy are incompatible with life and the fetus would spontaneously abort. Others, such as Down's Syndrome as mentioned above, result in various abnormalities for the affected individual. See the karyotype practice below to see some of these other conditions.

Here is an example of a normal karyotype1 .

Here is an example of Down's Syndrome2 - note the extra chromosome #21. This image also shows the two possible combinations of sex chromosomes (xx or xy).

External Links

Karyotype Explained - read this quick explanation of a karyotype and the 'p' and 'q' arms of the chromosome.
Karyotype Practice - Click on this link to practice matching chromosomes and to diagnose various conditions.

1 image courtesy of Wikimedia Commons
2 image courtesy of Wikimedia Commons

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