Genetic inheritance is a fundamental biological process where traits are passed from parents to their offspring through DNA. DNA, organized into chromosomes, contains the instructions that define an organism. Understanding how these instructions are shuffled across generations is important in genetic genealogy. A key concept in this field is the centimorgan, a unit that quantifies genetic distance. This article clarifies what a centimorgan is, explores its biological basis, and explains its importance in analyzing genetic relationships for ancestry.
What is a Centimorgan?
A centimorgan (cM) is a unit of genetic distance, not a physical measure. It quantifies the likelihood of genetic recombination between two points on a chromosome. One centimorgan signifies a 1% chance that two genetic markers on the same chromosome will be separated during meiosis in a single generation. A higher centimorgan value indicates a greater probability of recombination.
The centimorgan is a statistical measure. While not a direct physical distance, one centimorgan generally corresponds to about one million base pairs of DNA in humans. This physical correspondence can vary across different regions of the genome and between individuals. The unit is named after Thomas Hunt Morgan, a geneticist who contributed to the understanding of genetic linkage.
The Basis of Centimorgan Calculation: Genetic Recombination
Centimorgan calculation is based on genetic recombination, primarily driven by crossing over during meiosis. Meiosis is a cell division that produces gametes (sperm and egg cells), each with half the parent cell’s chromosomes. During meiosis, homologous chromosomes pair up and exchange DNA segments through crossing over, creating new genetic combinations.
The frequency of recombination events between two genetic markers determines centimorgan values. Markers close together on a chromosome are less likely to separate, resulting in a lower centimorgan value. Markers further apart have a greater chance of recombination, leading to a higher centimorgan value. These values are not typically calculated for a single individual directly. Instead, they are established statistically by observing recombination frequencies across many generations or large populations, which helps construct genetic maps showing the relative positions of genes on chromosomes.
Interpreting Centimorgan Values in DNA Matches
Centimorgan values are widely applied in consumer DNA testing to estimate genetic relationships. DNA testing companies report the total shared DNA, measured in centimorgans, between a test taker and their genetic matches. A higher shared centimorgan value indicates a closer genetic relationship. For example, a parent and child typically share 3330-3720 cM, while full siblings often share 2209-3384 cM.
Shared centimorgan totals predict possible relationships, with online tools and charts providing estimated ranges. However, a specific centimorgan value can correspond to multiple relationships due to the random nature of DNA inheritance. A shared amount could indicate a first cousin, a half-aunt, or a grandparent, necessitating further genealogical research to clarify the exact connection. The total shared centimorgans, along with the number and length of individual shared DNA segments, contribute to the strength of a DNA match.
Factors Influencing Centimorgan Values
Biological factors influence recombination rates, affecting centimorgan values. One factor is the difference in recombination rates between sexes. In humans, females generally have higher overall recombination rates than males, leading to longer genetic maps. For example, the female genome is estimated at 4460 cM, while the male genome is about 2590 cM.
Chromosomes contain regions known as recombination “hotspots” and “coldspots.” Hotspots are areas where recombination events occur with higher frequency, while coldspots are regions where recombination is significantly reduced. These variations mean that the physical length of DNA corresponding to one centimorgan can differ across the genome; for example, one cM in a hotspot might represent a smaller physical distance than one cM in a coldspot. The overall length of chromosomes also influences recombination. These biological variations contribute to the range of centimorgan values observed even among individuals with the same genealogical relationship.