First-degree relative

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A first-degree relative (FDR) is a person's parent (father or mother), full sibling (brother or sister) or child.[1] It constitutes a category of family members that largely overlaps with the term nuclear family, but without spouses.[2]

If the persons are related by blood, the first degree relatives share approximately 50% of their genes. First-degree relatives are a common measure used to diagnose risks for common diseases by analyzing family history.[3]

Marriage or sexual relations between first-degree relatives falls within the definition of incest.

If a parent has an identical twin, then that aunt or uncle who is the other twin, is also considered a first-degree relative because being genetically identical to the parent ultimately makes them the children's parent as well, genetically speaking.[citation needed]

See also[edit]


  1. ^ Talley, Nicholas (2007). Gastroenterology and Hepatology: A Clinical Handbook. p. 200.
  2. ^ Reiss, David (1981). The Family's Construction of Reality. p. 276.
  3. ^ Ginsburg, Geoffrey (2008). Genomic and Personalized Medicine, Volumes 1-2. p. 482.