A first-degree relative (FDR) is a person's parent (father or mother), full sibling (brother or sister) or child. It constitutes a category of family members that largely overlaps with the term nuclear family, but without spouses.
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.
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.
- Talley, Nicholas (2007). Gastroenterology and Hepatology: A Clinical Handbook. p. 200.
- Reiss, David (1981). The Family's Construction of Reality. p. 276.
- Ginsburg, Geoffrey (2008). Genomic and Personalized Medicine, Volumes 1-2. p. 482.