The centimorgan (cM) is the standard unit scientists use to measure genetic distance within the human genome. It does not represent a fixed physical length of DNA. Instead, the total cM count reflects the overall potential for genetic change across generations.
Defining the Centimorgan
A centimorgan is a unit of measure for the frequency of genetic recombination, which is the shuffling of DNA that occurs during the formation of egg and sperm cells, a process called meiosis. Specifically, one centimorgan is defined as the distance between two markers on a chromosome where there is a one percent chance that they will be separated due to a crossover event in a single generation. This unit is named after the geneticist Thomas Hunt Morgan, whose work on fruit flies established the concept of genetic linkage.
The centimorgan measures the likelihood of linkage between two genetic locations, not the physical length of the DNA strand. A higher cM value indicates a greater distance between markers, meaning they are less likely to be inherited together because there is a higher probability of recombination occurring between them. Although the relationship is rough, one centimorgan corresponds to approximately one million base pairs (one megabase, or 1 Mb) of physical distance in the human genome on average.
The physical length in base pairs for one centimorgan is not uniform across the entire genome. Different regions of a chromosome have varying propensities toward recombination; some areas are “hotspots” with high crossover rates, while others are “cold spots” with low rates.
The Total Genetic Map Length
The total genetic map length of the human genome is the comprehensive measure of all genetic distance in centimorgans across all chromosomes. This length represents the total potential for recombination in the genome. The established range for the total length of the human genetic map is approximately 6,000 to 7,000 cM.
This number is not a single, fixed value because the total genetic length varies significantly between the sexes due to sex-specific recombination rates. The female genetic map is substantially longer than the male map in centimorgans because recombination occurs more frequently during the formation of female gametes (oogenesis) than in the formation of male gametes (spermatogenesis). In one high-resolution study, the female autosomal genome was estimated to be around 4,782 cM, while the male autosomal genome was estimated to be around 2,809 cM.
When combining the autosomal chromosomes with the sex chromosomes, the total genetic map length for a female is generally reported to be around 4,460 to 4,782 cM, and for a male, it is between 2,590 and 2,832 cM. The sex-averaged value, which is often used in genetic studies and by DNA testing companies, falls in the middle of this range. This average leads to the overall total estimate of roughly 6,000 to 7,000 cM for the entire human genome.
Applying cM: Measuring Shared Ancestry
In the context of direct-to-consumer DNA testing, the centimorgan is the primary unit used to quantify shared ancestry and estimate relationship closeness. When two people take a DNA test, the company compares their genomes to find segments of DNA that are identical. The total length of these shared segments is reported in centimorgans, indicating the amount of genetic material inherited from a recent common ancestor.
The segments of shared DNA are referred to as Identity by Descent (IBD), meaning they were inherited from a shared ancestor within a genealogically relevant timeframe. The more centimorgans two individuals share, the more closely related they are likely to be. For instance, a parent and child share approximately 3,400 cM, while the amount shared with a second cousin is significantly less, falling into a much smaller range.
DNA testing companies use the total shared cM value to predict the probability of specific relationships, such as a first cousin versus a half-aunt. Because genetic inheritance is random, a single shared cM value can correspond to several different possible relationships, which is why the results are typically presented as a range of possibilities. The length of the longest shared segment is also a factor, as longer segments are more likely to be truly IBD and indicate a more recent common ancestor, since recombination rapidly breaks up long segments over generations.