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Human chromosomes
A chromosome is a thread-like structure containing most of the DNA of a living organism in the form of a double helix. A single chromosome may contain thousands of genes. Chromosomes are found within the nuclei of plant and animal cells, and contain most of the genetic information that makes each individual distinct. Humans have two sex chromosomes and 22 pairs of other chromosomes. The sex chromosomes come in two varieties, referred to as X and Y.

The sex chromosomes are the primary genetic factor determining the sex of an individual: a person with two X chromosomes (XX) will be genetically female, a person with one X and one Y (XY) will be genetically male. Some conditions can cause people's apparent gender to differ from their genetic sex.

There are also a number of more unusual chromosomal combinations, including:

  • XO: Turner Syndrome
  • XXY: Klinefelter syndrome
  • XYY;
  • XXYY
  • XXX
  • XXXX
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