Deoxyribonucleic acid (DNA) is the complex molecule organized into structures called chromosomes, which reside within the nucleus of nearly every cell. The full complement of an individual’s genetic material is the genome, and the vast majority of this information is contained within autosomal DNA. This DNA governs the development and function of all biological traits not directly related to determining sex.
The Structure and Location of Autosomes
Humans typically possess 46 chromosomes arranged in 23 pairs. Autosomes make up the first 22 of these pairs, numbered 1 to 22. These chromosomes are homologous, meaning the two members of each pair are generally identical in appearance and carry the same types of genes.
The autosomes are numbered approximately in order of size, with Chromosome 1 being the largest and containing the most genes. This DNA is confined to the cell’s nucleus, where it is tightly coiled around proteins to form the chromosome structure. Since autosomes are present in pairs, they carry the instructions for all physical and biochemical characteristics not primarily controlled by the sex chromosomes.
How Autosomal DNA is Inherited
The inheritance of autosomal DNA ensures every individual receives approximately 50% of their genetic material from each biological parent. This occurs when reproductive cells (gametes) are formed through meiosis. Before the DNA is packaged, the parental chromosomes undergo genetic recombination, or crossing over.
During recombination, the paired chromosomes exchange segments of DNA, effectively shuffling the genetic material. This creates a newly formed chromosome that is a patchwork of the grandparental DNA. Because this shuffling is a random event, each child inherits a unique combination of segments, which is why full siblings are genetically similar but not identical.
The effect of recombination is notable when looking at more distant ancestors. While a person receives 50% of DNA from each parent, the amount received from a specific great-grandparent is highly variable and often not the theoretical 12.5%. Due to the random nature of recombination, some segments of an ancestor’s DNA may be completely lost or retained in larger chunks, making the inheritance pattern unpredictable beyond a few generations.
Contrasting Autosomal DNA with Other Genetic Markers
Autosomal DNA is distinct from the two other main types of DNA used in genetic analysis: the sex chromosomes and mitochondrial DNA (mtDNA). The sex chromosomes (X and Y) determine biological sex and have a unilateral inheritance pattern. Females inherit two X chromosomes, while males inherit an X from their mother and a Y from their father.
The Y chromosome is passed down almost entirely unchanged, directly from father to son along the patrilineal line, making Y-DNA useful for tracing direct male ancestry. Mitochondrial DNA is found outside the nucleus in the cell’s energy-producing mitochondria.
Mitochondrial DNA is inherited exclusively from the biological mother by all her children, regardless of sex. mtDNA is passed down with very little change over many generations, allowing for the deep tracing of the direct maternal line. Autosomal DNA, by contrast, is inherited from all ancestors across all lines, making it the broadest genetic snapshot of recent family history.
Using Autosomal DNA for Ancestry and Relationship Mapping
The bilateral inheritance of autosomal DNA makes it the primary tool for consumer genetic testing, used for generating ethnicity estimates and mapping genetic relationships. Ethnicity estimates are calculated by comparing an individual’s DNA segments to reference populations worldwide. These comparisons identify segments that match particular populations, providing a percentage breakdown of ancestral origins across all lines.
Relationship mapping relies on identifying shared segments of autosomal DNA between two individuals in a testing database. The amount of shared DNA is measured in a unit called a centimorgan (cM), which is a measure of genetic distance that relates to the probability of recombination. The greater the total number of shared centimorgans, the closer the genetic relationship between the two people.
By quantifying the total cM shared, testing companies predict the proximity of the relationship, such as a first or third cousin. The nature of autosomal inheritance, including the shuffling caused by recombination, allows this broad-scope analysis to connect individuals to relatives across the entire family tree.