The phrase “sharing blood” refers to the genetic connection of deoxyribonucleic acid, or DNA, passed down through generations. DNA is the fundamental blueprint for life, connecting individuals to their relatives.
The Basics of Shared DNA
DNA is a complex molecule organized into chromosomes and individual units called genes. It consists of two long strands that twist around each other, forming a structure known as a double helix. These strands are composed of building blocks called nucleotides, which contain one of four nitrogenous bases: adenine (A), cytosine (C), guanine (G), and thymine (T). These bases pair specifically (A with T, and C with G), forming the “rungs” of the DNA ladder.
Humans inherit approximately 50% of their genetic material from each biological parent. While percentages offer a general idea of shared DNA, geneticists use a more precise unit called the centimorgan (cM) to measure genetic linkage and the length of shared DNA segments.
A centimorgan quantifies the likelihood that two genetic markers on a chromosome will be separated during meiosis, the process of forming egg and sperm cells. The more centimorgans two people share, the more closely related they are.
Your Genetic Link to Cousins
The amount of DNA shared with cousins depends on their relationship, which is defined by shared common ancestors. First cousins, for instance, share a set of grandparents. On average, first cousins share about 12.5% of their DNA, which typically translates to around 850 to 866 centimorgans. The actual amount can vary, usually falling within a range of approximately 396 to 1397 cM.
Second cousins share a set of great-grandparents. They generally share a smaller proportion of DNA, averaging about 3.125%. The typical range of shared centimorgans for second cousins is between 75 and 360 cM.
Third cousins share a set of great-great-grandparents. The average shared DNA for third cousins is even lower, typically around 0.78%. The amount of shared DNA for third cousins can range from as little as 6 cM up to about 50 cM.
Why Shared DNA Varies
The shared DNA percentages are averages rather than fixed amounts due to genetic recombination, also known as crossing over. During the formation of sperm and egg cells (meiosis), paired chromosomes exchange segments of DNA. This shuffling creates unique combinations of genes in each gamete, meaning every offspring receives a distinct blend of their parents’ genetic material.
Because of this random shuffling, even full siblings do not share an identical 50% of their DNA; it is an average, with the actual amount varying between approximately 38% and 61%. This variability is even more pronounced in more distant relationships like cousins. The precise segments and total amount of DNA inherited can differ significantly between individuals who share the same genealogical relationship.
Variations in cousin relationships also affect shared DNA. Half-cousins, for example, share only one common grandparent, as opposed to the two shared by full cousins. Consequently, half-cousins typically share about half the DNA of full cousins, averaging around 6.25% or between 215 and 650 cM. Double cousins, a rarer scenario where two siblings from one family marry two siblings from another family, share significantly more DNA than typical first cousins, often around 25%. This occurs because they effectively share two sets of grandparents.
What Shared DNA Reveals
Analyzing shared DNA segments provides insights into family connections. Genetic genealogy uses shared DNA to build and confirm family trees, helping individuals identify and connect with unknown relatives. DNA testing platforms report the amount of shared DNA in centimorgans, which can then be compared to known relationship averages to estimate kinship.
Beyond confirming family structures, shared DNA can also reveal information about inherited traits and predispositions to certain health conditions. For instance, understanding shared genetic material can be relevant in identifying risks for recessive genetic disorders within a family. Shared DNA also plays a role in broader ancestry research, allowing individuals to trace their ethnic origins and geographic migrations over generations. This scientific understanding of genetic inheritance offers a tangible basis for comprehending the deep bonds within families.