First-degree relatives

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. 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. Marriage between first-degree relatives is universally prohibited, as are sexual relations. If a parent is an identical twin, then the aunt or uncle who serves as 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.