What is a first cousin?
Your first cousin is the child of your aunt or uncle. You share a set of grandparents, and you are both two generations below them.
Preset to this relationship. Change the steps to compare with neighbouring relationships.
They are your
first cousin
Common ancestors: your grandparents. Expected shared DNA: 12.5%
Shorter branch = 2 steps → 2 − 1 = 1 → first cousin. Branches differ by 0 → not removed.
Your parent and your first cousin's parent are siblings. That is the whole test: if your parents are brother and sister (or brothers, or sisters), their children are first cousins.
First cousins share on average about 12.5% of their DNA (1/8), the same expected amount as with a great-aunt, a great-grandparent or a half-aunt. Real results vary because DNA is passed on in random chunks.
If your parents are only half-siblings, you share one grandparent rather than two and are half first cousins. If two siblings married two siblings, you share all four grandparents and are double first cousins.
Your first cousin's children are your first cousins once removed, and they are second cousins to your own children.
To check a real pair of relatives, add them to the family tree maker and use How related?, or look them up on the cousin chart.
Questions
Are first cousins blood related?
Yes. Full first cousins share two grandparents and about 12.5% of their DNA on average. Step-cousins, related only by marriage, share none.
What is the difference between a first cousin and a second cousin?
First cousins share grandparents; second cousins share great-grandparents. Your parents' first cousins' children are your second cousins.
Is my aunt's husband's nephew my cousin?
Not by blood. He is your aunt's nephew by marriage; most families would simply call him a cousin by marriage.