The term “sharing blood” is a colloquial expression referring to the genetic connection between individuals, specifically the amount of deoxyribonucleic acid (DNA) relatives have in common. This shared genetic material forms the biological basis for family relationships. Cousins, while not in a direct line of descent, share a common ancestor and thus some degree of DNA.
The Genetics of Family Ties
DNA, the blueprint of life, is passed down from parents to their offspring. Each child inherits approximately half of their DNA from their mother and half from their father. This inheritance involves a complex recombination of genetic material from each parent.
Siblings, for instance, share about 50% of their DNA on average, but the specific segments inherited can differ. This genetic variation explains why siblings can have different traits despite coming from the same parents. Shared DNA between relatives, including cousins, stems from their descent from a common ancestor.
As generations pass, the amount of shared DNA from a common ancestor typically decreases due to this recombination process. Each new generation introduces another round of mixing and matching DNA from two parents.
Quantifying Cousin Connections
The amount of shared DNA between relatives is often measured as a percentage or in centimorgans (cM). A centimorgan is a unit that estimates the likelihood of a DNA segment being passed down intact through generations, rather than splitting. The more centimorgans two individuals share, the more closely related they tend to be.
First cousins, who share a set of grandparents, typically share an average of 12.5% of their DNA, corresponding to about 866 centimorgans. The actual range can vary from approximately 396 to 1397 cM due to the random nature of genetic inheritance. When a cousin relationship spans different generations, the term “removed” is used. For example, a first cousin once removed is either your first cousin’s child or your parent’s first cousin, indicating a one-generation difference.
First cousins once removed generally share about 6.25% of their DNA. This amount is roughly half of what first cousins share, reflecting the additional generational distance. The average centimorgan range for first cousins once removed is approximately 102 to 980 cM.
Second cousins, who share a set of great-grandparents, typically share around 3.125% of their DNA, translating to about 246 centimorgans, with a general range from 47 to 760 cM. Third cousins, sharing a set of great-great-grandparents, have an even smaller average percentage of shared DNA, around 0.78%. The amount of DNA shared with third cousins can range from as little as 0.06% to 2.2%, or 0 to 75 cM, and in some cases, third cousins may share no detectable DNA at all.
Why Shared DNA Matters
The shared DNA among cousins carries implications for both health and understanding family history. When closely related individuals, such as cousins, have children, there is an increased likelihood of sharing the same recessive gene variants inherited from common ancestors.
Recessive genetic conditions only manifest if an individual inherits two copies of a mutated gene, one from each parent. If cousins have children, the probability of both parents carrying the same recessive mutation is higher than in the general population, which can elevate the risk for their offspring to inherit such conditions. Studies suggest that children of first cousins may have a risk of recessive genetic disorders that is about double the risk for children of unrelated parents, typically increasing from a 2-3% baseline to 4-6%.
Beyond health, shared DNA is a powerful tool in ancestry and genealogy. DNA testing allows individuals to identify genetic relatives and estimate their relationship based on the amount of shared DNA. This can help in tracing family lines, confirming known relationships, and even discovering previously unknown relatives or common ancestors. By comparing DNA segments and the number of shared centimorgans, genealogists can build more comprehensive family trees and connect with distant relatives.