Blogging the Human Genome

The only break in the monotony comes with chromosome 21. You?d think this would be the 21st-longest, but no?it?s the 22nd-longest, and the shortest chromosome overall, roughly 3 million DNA bases shorter than chromosome 22. Back when scientists started cataloging human chromosomes, they had much poorer microscopes, and a difference of even many millions wasn?t easy to measure. That?s because chromosomes in living cells usually look like unraveled skeins of yarn, not the neat, plump, paper-doll pairs we?re used to seeing on karyotypes (like the snazzy one below). During this era, scientists mistakenly measured chromosome 21 as longer, and by the time they realized this, it had already become infamous as the cause of Down syndrome. (Down syndrome occurs when someone has three copies of chromosome 21 instead of the normal two.) Rather than risk confusion, scientists let the misnomer ?21? stick.

Source: http://feeds.slate.com/click.phdo?i=3474113770ccd27274b5c4c02040436c

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