A trisomy – simply put, is when an individual has an extra chromosome (three, instead of the usual two-pair).
Chromosomes are “thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). Passed from parents to offspring, DNA contains the specific instructions that make each type of living creature unique.” (source: genome.gov)
Humans typically have 23 pairs of chromosomes, for a total of 46. A trisomy is when one particular pair has an extra chromosome, making three instead of two of a particular one.
The effect of a trisomy can vary widely, depending on which chromosome has the extra. For example, a girl with trisomy 23 (an extra X chromosome) would show little to no effect from it. Compared to babies born with trisomy 18, which has a high infant mortality rate.
Down Syndrome is trisomy 21, and is the most common chromosome disorder in the United States.
The 22 autosomes are numbered by size. The other two chromosomes, X and Y, are the sex chromosomes. This picture of the human chromosomes lined up in pairs is called a karyotype. This particular karyotype shows a female with Down Syndrome.