Degree of relationship is a measurement of kinship, and may generally be measured as either one vertical or horizontal step in a standard family tree.
A first-degree relative is a family member who shares about 50 percent of their genes with a particular individual in a family.[1] First degree relatives include parents, offspring, and siblings.[1] An uncle/aunt, niece/nephew, grandparent, or half-sibling is a second-degree relative and shares 25 percent of their genes. A first cousin is a third-degree relative and shares 12.5 percent of their genes.[2] Degrees of relation should not be confused with degrees when referring specifically to kinship with cousins. For example, a first-degree cousin is actually a third-degree relative.
It is an important measurement in law to determine where marriage is allowed or prohibited under the laws concerning incest.[3]
It is also an important measure in medicine when determining risk factors for various diseases with a strong hereditary association.
It is also used in genealogy (especially genetic genealogy) to measure the level of genetic relationship between individuals.