A first-degree relative is a family member who shares approximately 50% of their DNA with you. This includes your parents, your full siblings (brothers and sisters), and your children. The term comes up most often in medical settings because these are the relatives whose health history has the strongest implications for your own risk of disease.
Who Counts as a First-Degree Relative
The list is short and specific. Your first-degree relatives are:
- Parents: your biological mother and father
- Siblings: your full brothers and sisters
- Children: your biological sons and daughters
Each of these people shares roughly half of your genetic material. That 50% overlap is the defining feature, and it’s what separates first-degree relatives from more distant family. Half-siblings share only about 25% of DNA, which places them in a different category.
How Other Degrees Compare
Second-degree relatives share about 25% of your DNA. This group includes grandparents, grandchildren, aunts, uncles, nieces, nephews, and half-siblings. Third-degree relatives, like first cousins, share roughly 12.5%. The further out you go on the family tree, the less genetic overlap exists, and the less predictive a relative’s health history becomes for your own.
Doctors care most about first-degree relatives because that 50% genetic overlap means shared risk for inherited conditions is at its highest. A grandparent’s diagnosis still matters, but it carries less statistical weight than the same diagnosis in a parent or sibling.
Why Doctors Ask About First-Degree Relatives
When you fill out a family history form at a doctor’s office, questions about first-degree relatives aren’t just routine paperwork. They directly shape your screening schedule and risk profile. Many conditions cluster in families due to shared genes, shared environment, or both, and a diagnosis in a parent or sibling often doubles your own risk.
Breast cancer is one well-studied example. Having a mother, sister, or daughter diagnosed with breast cancer approximately doubles your risk compared to someone without that family history. The risk climbs higher when more than one first-degree relative has been diagnosed, or when any of them were diagnosed before age 50.
Cardiovascular disease follows a similar pattern. If a parent or sibling had a heart attack or coronary artery disease at a young age (before 55 for men, before 65 for women), your own risk roughly doubles. That age cutoff matters: early-onset disease in a close relative is a stronger signal of inherited vulnerability than disease that develops later in life.
How Family History Changes Cancer Screening
Colorectal cancer screening guidelines shift significantly based on first-degree relative history. For the general population, routine screening typically begins at age 45. But if you have one first-degree relative who was diagnosed with colorectal cancer at any age, guidelines recommend starting screening at age 40 to 50, or 10 years before the age your relative was diagnosed, whichever comes first. Colonoscopies in this group are repeated every 5 to 10 years rather than the standard interval.
If two or more first-degree relatives had colorectal cancer, the timeline tightens further. Screening starts at age 40 or 10 years before the youngest relative’s diagnosis, and colonoscopies are repeated every 5 years. The key detail here: these adjusted timelines apply specifically to cancer or advanced precancerous growths in a first-degree relative. If a parent had small, non-advanced polyps, standard screening guidelines still apply.
Similar adjustments exist for breast cancer, ovarian cancer, and prostate cancer screening. In each case, a first-degree relative’s diagnosis can trigger earlier screening, more frequent monitoring, or additional testing like genetic counseling.
What Counts as “Family History”
Your family medical history, for clinical purposes, means the health conditions of your biological relatives. Adoptive parents, stepparents, and in-laws don’t factor into genetic risk assessment, even though they may influence your environment and habits. If you were adopted or don’t have access to your biological family’s health records, it’s worth mentioning that gap to your doctor so they can consider other ways to evaluate your risk.
The most useful details to know about a first-degree relative’s health include which conditions they were diagnosed with, how old they were at diagnosis, and whether the condition was on your maternal or paternal side. Age at diagnosis is particularly important because earlier onset often signals a stronger genetic component.
Legal Protections for Genetic Information
Because family medical history is so revealing, federal law offers some protection against its misuse. The Genetic Information Nondiscrimination Act (GINA) makes it illegal for employers to discriminate based on genetic information, which explicitly includes your family members’ medical history. This covers hiring, firing, promotions, pay, and job assignments. Employers generally cannot request or require your family health information, and harassment based on a relative’s genetic information is also prohibited.
GINA also restricts how health insurers can use genetic information when making coverage and pricing decisions. However, the law does not extend to life insurance, disability insurance, or long-term care insurance, which remain gaps in protection.