At-home genetic testing allows individuals to measure their relationships with distant relatives, often raising the question of how much deoxyribonucleic acid (DNA) they share with their cousins. The amount of shared genetic material is a quantifiable measure of biological connection, usually expressed as a percentage. Determining the precise degree of cousin relationship can be complex because the actual amount of shared DNA varies widely, even for relatives with the same genealogical title. The data provided by genetic testing companies are statistical probabilities that help map out the branches of a family tree.
Defining Genetic Distance Measurement
Genetic testing companies quantify shared DNA using the CentiMorgan (cM), a unique unit of genetic distance. The CentiMorgan is not a physical measure of length, but reflects the likelihood of recombination. One CentiMorgan represents a one percent chance that a segment of DNA will separate and swap with its corresponding segment during the formation of reproductive cells in a single generation. This unit measures the probability of genetic material being passed down intact from a common ancestor.
The CentiMorgan is used because DNA is inherited in large, recombined chunks, making it a more accurate measure of shared ancestry than a simple base pair count. The human genome is estimated to be approximately 6,800 CentiMorgans, which provides the baseline for calculating shared percentages. For example, sharing 68 cM of DNA means approximately one percent of the total genome is shared. The more CentiMorgans two individuals share, the more recent their common ancestor is likely to be.
Expected DNA Sharing by Cousin Relationship
The average amount of shared DNA decreases predictably with each step further away from a common ancestor, providing a useful guideline for predicting relationships. Full first cousins, who share a pair of grandparents, are expected to share an average of 12.5% of their DNA. This percentage typically translates to a CentiMorgan range between 530 cM and 1,128 cM, with the average falling around 859 cM. Even with this close relationship, the inheritance process introduces enough variability that one pair of first cousins may share significantly more or less DNA than another pair.
Moving one step further to second cousins, who share a pair of great-grandparents, the expected average shared DNA drops to approximately 3.125%. This lower percentage corresponds to a wide CentiMorgan range, typically between 73 cM and 383 cM, with an average of about 233 cM. The variability is noticeably greater at this level, meaning a second cousin could genetically appear as close as a first cousin once removed due to random inheritance.
Third cousins share a pair of great-great-grandparents, and the average shared DNA is expected to be around 0.78%. The actual CentiMorgan count for third cousins is highly variable, often ranging from 0 cM up to nearly 200 cM, with a crowd-sourced average closer to 79 cM. The chance of not sharing any measurable DNA segments increases significantly at the third cousin level and beyond.
The term “removed” in a cousin relationship indicates a difference in the number of generations separating the two individuals from their common ancestor. A first cousin once removed, for example, is separated by one generation from the first cousin relationship, such as the child of a first cousin. The expected shared DNA for a first cousin once removed is approximately 6.25%, with a CentiMorgan range typically found between 218 cM and 638 cM. This generational difference means the amount of shared DNA is halved compared to the full first cousin relationship.
Why Shared DNA Amounts Can Vary
The reason the shared DNA amounts are expressed as a wide range rather than a fixed number is primarily due to the biological process of random recombination. During the formation of a parent’s reproductive cells, their chromosomes swap segments of DNA from their own parents in a random shuffling process. While a child inherits precisely 50% of their DNA from each parent, the specific mix of segments inherited from each grandparent is unpredictable. Consequently, two full siblings do not inherit the exact same combination of their parents’ DNA, leading to differing amounts of shared DNA with the same cousins.
The amount of shared DNA can also be significantly affected by a family relationship known as a “half-cousin.” A full first cousin shares both grandparents, but a half-first cousin shares only one grandparent, which typically halves the expected shared DNA. For a half-first cousin, the average shared DNA falls around 4.5% to 5%, with CentiMorgan totals ranging from approximately 215 cM to 650 cM. This overlap in CentiMorgan ranges with full second cousins or even first cousins once removed often requires additional genealogical information to accurately determine the exact relationship.
Two more complex genealogical factors that affect shared DNA are pedigree collapse and endogamy, both of which can artificially inflate CentiMorgan counts.
Pedigree Collapse
Pedigree collapse occurs when two related individuals have children together, causing one or more ancestors to appear multiple times in a family tree. This results in the descendant inheriting a larger portion of DNA from those duplicated ancestors, making a close relative, such as a second cousin, appear genetically closer than expected.
Endogamy
Endogamy refers to a pattern of intermarriage within a small, isolated population over many generations. In endogamous groups, individuals will share numerous small segments of DNA with distant relatives due to the limited gene pool. This leads to a higher total CentiMorgan count that does not reflect a single, recent common ancestor.