Glossary - DNA

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The following terms include those that will be most frequently encountered at an early stage in a study of genetic genealogy.


An allele is an item of genetic information within a gene. In a genealogical DNA test it is almost invariably expressed as a whole number. This allele value is simply the count of the number of base pair tandem repeats found at a particular STR marker (and sometimes adjusted according to agreed norms). Each value will fall within a tight range of values that are associated with each marker. An allele changes through mutation, but only rarely.

Atlantic Modal Haplotype

Name given to the most common haplotype identified in large-scale British and European Y-chromosome testing projects. Not used so often today as haplotype resolutions have increased.

Autosomal DNA 

DNA found in the 22 pairs of human chromosomes, i.e. other than in the sex-determining 23rd chromosome and the mitochondrial DNA. A mother's and father's autosomal DNA is subject to recombination, or genetic shuffling, during the process of creating offspring. Many heritage and deep ancestry tests target the autosomal DNA, and it is the focus of forensic genetic tests that seek to determine unique identities.

Cambridge Reference Sequence (rCRS) 

Reference definition of the DNA signature of human mitochondrial DNA, first published in 1981 and revised last in 2009. 


The basic unit of life: a miniature factory producing the raw materials, energy and waste removal capabilities necessary to sustain life. The average human has around 100 trillion cells, each of which contains much the same human genetic programming.


Made of DNA and special proteins, chromosomes are the receptacle for genes which are arranged along their length in sections known as coding regions. Humans have 23 pairs of chromosomes, one of each pair from their mother and one from their father.

Clades or subclades

Branches of haplogroups, defined by single nucleotide polymorphisms (SNPs).

Deep ancestry 

Ancestry prior to the beginning of the standard genealogical timeframe (the period when hereditary surnames came into effect), and stretching back to mankind's origins in Africa.


Deoxyribonucleic acid, the read-only memory of the genetic information system that resides within each cell and which contains the coded instructions required to replicate the cell and associated enzymes.

DNA signature 

Shorthand phrase to describe the haplotype that belongs to an individual DNA study participant or to a group of participants. A DNA signature can exist at different resolutions.

DNA test 

Within a genealogical context this can refer to a multiple-marker Y-chromosome test, mitochondrial DNA tests and autosomal DNA tests.


Acronym for DNA/Y-chromosome/Single-copy sequence. Each DYS marker includes a number which indicates the locus number in sequence of discovery. Other locus descriptions also exist  e.g. GGAAT1B07  which describe the actual location of the locus on the chromosome. These subsequently may be given an official DYS sequence number.


The discrete messages contained in the chromosomes that produce all the differences between us. Humans have an estimated 30,000 genes, only several times more than the humble fruit fly.

Genetic bottleneck

When a human population undergoes a migration, only a few members of the original population survive to recreate the group in the new location. This surviving group may then breed and come to outnumber the original population whilst containing less genetic diversity than it. A good example is the UK-derived population of the USA, which is many times larger in numbers but is made up of only a fraction of the genetic diversity originally present in the UK.

Genetic clan 

A shorthand phrase to describe members of a clan that share a common genetic heritage through identical, or near identical, haplotypes. Popular science writers also use the phrase to describe haplogoups and the heads of specific genetic lineages, sometimes giving them fictional identities.

Genetic family 

Shorthand phrase to define men that have an identical, or near identical, haplotype or DNA signature and who share a common surname. The genetic family is what you create when you aggregate similar Y-chromosome DNA results together.

The branch of biology that deals with heredity, the mechanisms of hereditary transmission and the variation of inherited characteristics among related organisms.


Large-scale groups of humans identified in studies of ancient human migrations. Defined by differences seen in a specific set of SNP markers. Haplogroups are becoming increasingly well defined down in some cases to five or more levels of sub-groups.


A string of numerical values derived from different markers that collectively make up the result of a Y-chromosome test and define your DNA signature. Haplotypes can consist of any number of markers and therefore exist at different marker resolutions.


The position in the DNA where its structure is measured during a DNA test.

Mitochondrial DNA (mtDNA) 

Often described as the cell's battery as it performs the essential function of creating, storing and transferring energy within the cell. The genetic code in mitochondrial DNA is passed across the generations from mother to daughter, though every one of us has our mother's mtDNA.


A statistical term that describes the most commonly found value among a range of values. In genetic genealogy, often applied to the DNA signatures that can be described the most frequently found among a range of samples.

Most Recent Common Ancestor (MRCA) calculation 

This attempts to quantify the number of generations since two DNA haplotypes diverged from a common ancestor. Potentially this point could lie at any time since the first humans migrated from Africa. The calculation is highly inexact. The results of an MRCA calculation are expressed in terms of a percentage probability (usually 50% or 90%) plus the range of generations in which the result is likely to lie. The calculation is more useful for clan studies which have a longer timeframe than for surname-oriented genealogists who are generally concerned with a relatively short 500-year timeframe.


DNA alters through mutation, which is a slight change in its molecular structure as measured at a particular marker. In terms of the test result the marker value will be slightly different after a mutation than it would have been prior to it, for example 14 instead of 13 or 10 instead of 11. Such a change is described as a single-step mutation, while a change from 12 to 14, for example, would be described as a double-step mutation.

Non-paternity event (NPE)

Also known as false paternal event, misattributed paternity or non-patrilineal transmission. A break in the Y-chromosome line resulting from a number of different scenarios: illegitimacy, fostering, formal/informal adoptions or name change and change of surname, both before the establishment of heridatary surnames and as a result of inheritance of land and/or titles.

Out of Africa thesis 

Theory, now almost universally accepted, that mankind's origins lie in Africa. Debate still rages, however, about whether the rest of the world was populated by a single migration from Africa or multiple migrations over an extended period of millennia. Built originally around archaeological and paleo-anthropological findings about extinct human species, the theory was given a huge boost by a ground-breaking paper published in Nature by Rebecca Cann in 1987 and subsequently by hundreds of other papers reporting mtDNA and Y-chromosome results.


A term in genetics that is an abbreviation for "Genetic Recombinational Loss of Heterozygosity". In genetic genealogy, the term is used particularly concerning similar seeming events in Y chromosome DNA. This type of mutation happens within one chromosome, and does not involve a reciprocal transfer. Rather, one homologous segment "writes over" the other.


An elegant and efficient method for our genes to experiment with gene adaptation without leaving this solely up to chance mutation. The process allows the two strands in the DNA's double helix to unravel during the replication phase of the reproduction process and then to recombine and include genetic data from a second source. From the genealogist's point of view recombination is a problem as it effectively jumbles up any offspring's DNA message. The DNA tests used by genealogists target the Y-chromosome which does not recombine during replication.


Every haplotype is defined by its degree of resolution. A 4-marker haplotype is not as well defined as a 67-marker haplotype. Genealogists should avoid low-resolution Y-chromosome tests and at a minimum chose a medium resolution test of at least 37 markers.


A statistical term and in this context to be used by non-statisticians with care. Genetic genealogists will come across it primarily when reviewing MRCA calculation results which are expressed in terms of different confidence levels. However, it also encompasses the art of describing to two project participants whether their results match or not.

SNP markers 

Abbreviation of single nucleotide polymorphism markers. At this type of marker mutation occurred only once at a specific position in a particular chromosome in a single individual. SNPs are like an on/off switch that was thrown at a particular moment in time and has not changed since. When you are DNA-tested on an SNP marker you will have either result A or result B. The group of descendants that each SNP value defines is known as a haplogroup.

STR markers 

Abbreviation of short tandem repeat markers.The standard Y-chromosome tests sold to genealogists all measure STR markers. Test companies like these markers because in the lab they can test for several STRs at a time. Geneticists like them because there are many of them and some are fast-mutating while others are slow to mutate.


The sex-determining chromosome that is present only in males. Mimics the father-to-son transmission of surnames as its DNA is handed down without the shuffling of recombination. Surname-based DNA studies should use only Y-chromosome DNA tests for their male participants.

Y Chromosome Consortium (YCC) 

Collaborative effort by geneticists to agree a way to describe Y-chromosome haplogroups. Its results were defined in a paper published in Genome Research in 2002 called A Nomenclature System for the Tree of Human Y-Chromosomal Binary Haplogroups. It periodically upgrades its work on its website, most recently in 2008.