first-degree relative
Meaning of first-degree relative: The closest ring of biological family, parents, siblings and children, who share about half their genes with a given person.
Definition of first-degree relative
A first-degree relative is a parent, a full brother or sister, or a child of the person in question. Members of this group share on average about half of their genes with that person. The category is used when assessing inherited risk, because a condition appearing in this closest ring carries more weight than the same condition in a more distant relative.
How are the degrees counted?
Degree reflects the proportion of genes shared, and it halves at each step outward.
| Degree | Who it includes | Genes shared |
|---|---|---|
| First | Parents, siblings, children | About one half |
| Second | Grandparents, aunts, uncles, nieces, nephews, half siblings | About one quarter |
| Third | First cousins, great-grandparents | About one eighth |
Identical twins are the exception that breaks the pattern, sharing effectively all of their genes despite being first-degree relatives to one another.
Why does the category matter?
Family history is one of the few risk factors that cannot be changed, and it is only useful if collected precisely. Guidance for screening frequently turns on whether a first-degree relative was affected and at what age, since disease appearing unusually early in a close relative suggests an inherited contribution rather than chance. Counting relatives correctly therefore changes what is offered. The distinction also shapes how relative risk figures are quoted, because a risk stated for one degree of relationship does not apply to another.
Where is it commonly misunderstood?
Adoptive and step relatives are close family in every social sense but share no genes, so they do not count for this purpose. Half siblings share one parent rather than two and fall into the second-degree group. Relatives by marriage never count. The relevant relationships must also be traced through both sides of the family, since a condition passing through the father is as significant as one passing through the mother for most inherited risks. A family history is recorded as who was affected, how they are related, and how old they were when diagnosed, because all three pieces are needed to interpret it.
Also known as
Used in a sentence
The family history recorded colorectal cancer in a first-degree relative diagnosed at a young age.
Good to know
Adoptive, step and in-law relatives share no genes and do not count toward family history, while half siblings fall into the second-degree group rather than the first.