18 Chromosome represents about 2.5% of the total DNA in cells. People normally have two copies of this chromosome as all other non-sex chromosomes. Chromosome eighteen spans about 76 million base pairs. Base Pairs are two molecules (nucleotides) on opposite DNA strands that are connected. Adenine (A) forms a base pair with thymine (T), as does guanine (G) with cytosine (C) in DNA. Base pairs is how DNA is measured. There are about 432 genes in this chromosome.

Trisomy 18 is the second most common trisomy and occurs when a baby has three of the 18 chromosome. This results in 47 chromosomes instead of the normal 46 in the affected cells. It is this extra genetic material that causes the problems associated with Trisomy 18.

Trisomy 18 is also called Edwards syndrome (or Edward's syndrome) and occurs in about 1:3000 live births. Unlike Down syndrome, Trisomy 18 is usually fatal, with most of the babies dying before birth and those who do make it to birth typically living only a few days. However, a small number of babies (<10%) live at least one year.

This is also the chromosome that involves club feet.

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18 Chromosome To 19 Chromosome