A DNA segment is a continuous stretch of deoxyribonucleic acid (DNA), serving as the blueprint for life. It represents a defined portion of DNA on a chromosome, marked by a beginning and an end point. This piece of genetic information can vary greatly in length, from just a few hundred base pairs to millions. Understanding them helps comprehend how genetic information is organized and utilized within an organism.
The Building Blocks of a DNA Segment
DNA segments are part of chromosomes, coiled structures found in the nucleus of most cells. Humans typically have 23 pairs of chromosomes, totaling 46, with one set inherited from each parent. Each chromosome is a very long strand of DNA tightly wound around proteins.
Within these DNA strands, segments are identified. Some segments are genes, containing instructions for creating proteins or functional RNA molecules. These proteins perform most of the work in cells and are necessary for the structure, function, and regulation of the body’s tissues and organs. However, a substantial portion of DNA, over 98% in humans, does not code for proteins. These non-coding segments can still have important roles, such as regulating gene activity, or their functions are not yet fully understood.
Inheritance and Genetic Recombination
DNA segments are passed from parents to children through a process that introduces genetic variation. Every person receives approximately half of their DNA from each parent. Before genetic material is passed on, genetic recombination occurs during egg and sperm cell formation. This involves the exchange of genetic material between the two copies of chromosomes a parent inherited from their own parents (the child’s grandparents).
During recombination, homologous chromosomes (pairs inherited from each parent) align and exchange DNA segments. This “shuffling” ensures that each gamete (sperm or egg cell) produced contains a unique combination of genetic information from both of the parent’s ancestral lines. As a result, children inherit long, continuous DNA segments from their parents, but the precise combination is unique to each child. This means full siblings (except identical twins) will not have the exact same DNA segments. This random assortment and recombination of segments explain the genetic diversity observed within families.
Shared DNA Segments in Genetic Genealogy
In DNA testing, a “shared DNA segment” or “matching segment” is a continuous stretch of DNA two individuals have in common. These segments indicate inheritance from a common ancestor. The amount and length of these shared segments are measured using a unit called a centiMorgan (cM).
A centiMorgan (cM) is a unit of genetic linkage, representing the probability of recombination between two points on a chromosome. A 1 cM distance indicates a 1% chance of a crossover event between those two points per generation. Genetic genealogists use the total shared cM across matching segments to estimate relationship closeness. A higher total cM count suggests a more recent common ancestor and a closer familial bond. For example, parent-child relationships typically share around 3500 cM, while full siblings share an average of 2613 cM, ranging from approximately 1613 cM to 3488 cM.
Interpreting Shared Segment Length
Beyond total shared DNA, individual segment length provides clues about relationships. Longer, continuous shared segments generally suggest a more recent common ancestor. This is because genetic recombination, the shuffling of DNA during inheritance, tends to break down longer segments into shorter ones over successive generations. Therefore, a very long shared segment, for example, over 40 cM, often points to a close relationship, such as a third cousin or closer.
Conversely, very short segments (below 7-10 cM) are less reliable for establishing recent genealogical connections. These short segments have a higher chance of being “identical by state” (IBS) rather than “identical by descent” (IBD). IBS means the segments are identical by chance due to random mutations or common population-level ancestry, not because they were inherited from a recent common ancestor. Such segments are often found in “pile-up regions,” specific chromosomal areas shared by many unrelated individuals. These regions can lead to false positive matches, so focusing on larger segments and using other genealogical evidence is recommended to confirm relationships.