Chromosome 21 facts for kids

Chromosome 21 is one of the 23 pairs of chromosomes found in humans. Think of chromosomes as tiny packages inside your cells that hold all your DNA. Chromosome 21 is quite small, holding about 48 million base pairs. These base pairs are like the building blocks that make up your DNA, and this chromosome contains about 1.5 percent of all the DNA in your body's cells. It's the smallest human autosome, which means it's not one of the chromosomes that decides if you are a boy or a girl.

Most people have two copies of each chromosome, including chromosome 21. However, sometimes a person can have an extra copy of chromosome 21.

What is Down Syndrome?

If someone has three copies of chromosome 21 instead of the usual two, they have a condition called Down syndrome. This is also known as trisomy 21 because "tri" means three. People with Down syndrome have unique physical features and may have different learning abilities.

Discovering Chromosome 21

Scientists have been working hard to map out all the human chromosomes. In May 2000, researchers involved in the Human Genome Project made an exciting announcement. They had successfully identified the complete sequence of base pairs that make up chromosome 21. This was a big step in understanding human genetics! Chromosome 21 was actually the second human chromosome to have its entire sequence mapped out. The first one was Chromosome 22.

Images for kids

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  This picture shows a karyotype, which is like a map of someone's chromosomes. You can see an extra copy of chromosome 21, which is what causes Down syndrome (also called Trisomy 21).

See also

In Spanish: Cromosoma 21 (humano) para niños