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What are Shared DNA segments?

Shared DNA segments, also referred to as ‘matching segments’, are the sections of DNA that are identical between two individuals. These segments were most likely inherited from a common ancestor.

DNA segments can be found on all of the 22 autosomal chromosomes. The segment length is determined by the centiMorgan distance between the first SNP and the last SNP. The longer the shared segment is, the higher the probability that it was inherited from a common ancestor, which means that the two people are genetically related.

All 22 pairs of chromosomes add up to a total of about 7000 centiMorgans. Half is inherited from your mother, and the other half from your father. 

You can use the following range of average values as a reference for the length of shared segments between you and your relatives:

  • Identical twin: 7000cM 
  • Parents: 3350 - 3600 cM
  • Full siblings: 2300 - 2900cM 
  • Grandparents and aunts/uncles: 1300 - 2200cM 
  • First cousins: 600 - 1200cM

Keep in mind that the above values represent average ranges. In some cases matches can have lower or higher centiMorgan values.

You might be interested to read: How do I use the Chromosome Browser for shared DNA segments?