Deoxyribonucleic acid, or DNA, serves as the fundamental blueprint for all living organisms, carrying unique instructions for each individual. While humans share over 99% of their DNA, the specific arrangement of the remaining small percentage accounts for individual differences and familial resemblances. Shared DNA decreases with each generation, making it a valuable tool for understanding genetic relatedness within families.
How DNA is Passed Down
Humans inherit DNA from both parents, roughly half from each. This occurs via specialized reproductive cells called gametes (sperm in males, eggs in females). Before gamete formation, meiosis divides parent cells to produce cells with half the chromosomes. During meiosis, genetic recombination, also called crossing over, shuffles DNA segments between homologous chromosomes, creating new combinations. This random assortment and recombination ensure that each gamete, and subsequently each offspring, receives a unique blend of parental DNA, contributing to genetic diversity.
Shared DNA with Cousins
The amount of DNA shared with cousins varies by relationship closeness, measured in centimorgans (cM), a unit reflecting genetic linkage. First cousins, sharing a set of grandparents, average 12.5% of their DNA, or about 866 cM, with a range of 396 to 1397 cM. First cousins once removed, a generation apart, average 6.25% of their DNA, or about 433 cM, ranging from 102 to 980 cM.
Second cousins, sharing a set of great-grandparents, average 3% of their DNA, or about 246 cM, with a range of 46 to 515 cM. Third cousins, sharing a set of great-great-grandparents, average 0.78% to 0.8% of their DNA, or about 79 cM, ranging from 0 to 198 cM. Beyond this relationship, shared DNA becomes often undetectable through standard DNA testing.
Why Shared DNA Amounts Differ
While average shared DNA percentages provide a guideline, the actual amount of DNA shared between relatives of the same type can differ. This variation arises from the random nature of DNA inheritance during meiosis. Each time DNA is passed from parent to child, recombination shuffles the genes, meaning that siblings, for example, do not inherit identical halves of their parents’ DNA. Therefore, two first cousins might receive slightly different segments from their shared grandparents, leading to a range of shared DNA rather than a fixed amount. Half-cousin relationships or situations where individuals are related through multiple ancestral lines can also influence the total amount of shared DNA.
What Shared DNA Reveals
Understanding shared DNA has practical applications, particularly in genetic genealogy. DNA testing services analyze these shared segments to estimate the degree of relatedness between individuals. By comparing DNA profiles, these services can identify potential relatives and suggest their likely relationship, often presented in centimorgans. This information helps individuals construct or verify their family trees and connect with previously unknown relatives, offering insights into their family history and ancestral origins.