Relatives that share 25% of their DNA, for example a grandparent and grandchild, half siblings, aunt/uncle and niece/nephew.
Use in clinical context
Individuals who are related share DNA as it is passed on to each subsequent generation at fertilisation. The percentage of DNA that individuals in a family share varies depending on the degree of relatedness – closer relatives share more DNA than more distant relatives.
Due to recombination second degree relatives may share very little of their DNA, or a large amount, but on average they share 25% of their DNA.
Second degree relatives in a genetic family history are identified as those individuals with one other family member between them e.g. grandparent and grandchild, half siblings, aunt/uncle and niece/nephew.