DNA glossary for family history

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28 November 2019
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Get to grips with genetic genealogy with our DNA glossary, written especially for anyone using DNA for family history.

DNA has a language of its own and it’s one you might not be familiar with. It’s worth getting to grips with some of the most commonly used words and terms – some are scientific and others have evolved and are specific to the genealogy community.

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Autosome (atDNA or auDNA)

An autosome is a chromosome, other than the sex chromosomes. Humans typically have 22 pairs of autosomes and one pair of sex chromosomes (allosomes).

Admixture

The more correct term for ‘ethnicity report’. Each of our admixture refers to our geographic origins as interpreted by our DNA.

Alleles

A specific form of a gene. Our genes have the capacity for a number of different outcomes. The allele is that outcome. Eg the gene may does to give us a trait, such as brown eyes.

Base pair (bp)

A rung on the DNA ladder, each base pair is made up of a pair of chemicals. The chemicals are adenine (A), which pairs with thymine (T) and guanine (G), which pairs with cytosine (C). The information about your base pairs is included in your raw DNA data.

centiMorgan

A unit for measuring genetic linkage. From a genealogy point of view the importance lies in a number of centiMorgans you share with a match – more centiMorgans tend to mean a closer relationship (the number of shared centiMorgans can vary within ballpark parameters).

Chromosomes

These are found inside the nucleus of a cell. Inside each chromosome is DNA. Humans typically have 23 pairs of chromosomes.

Chromosome browser

A tool displaying which segments of chromosome you and a DNA match share. 23andMe, Family Tree DNA, MyHeritage and DNAPainter all provide chromosome browser tools.

FIR fully identical region

Identical segments of chromosomes between you and a match. The DNA of siblings typically comprise 25% FIRs; identical twins have 100% FIRs.

Genes

These are made of DNA. Humans each have about 24,000 genes.

Haplogroup

The group to which you are assigned, based on your match to a common ancestor on your maternal and/or paternal lines. (Males can be assigned to two haplogroups – mother’s side and father’s side; females can just be allocated to a haplogroup on their mother’s side). Each haplogroup is given a name consisting of a letter and number combination.

IBD identical by descent

An identical segment between two or more matches of DNA inherited from a common ancestor.

IBS identical by state

Identical segments aren’t shared with a common ancestor, so are just a false positive. IBP (identical by population) is an identical segment of DNA that is shared by many people in a population group (ie also no useful for precise family history purposes).

NPE non paternity event

A DNA result that reveals and unexpected proportion of DNA between a child and its parent. Eg a child gets 50% of DNA from each biological parent (and share about 3,400 centiMorgans with each parent). A non-paternity event several generations ago will still be reflected in the DNA inheritance f the offspring with half relationships.

Phylogenetic tree

A diagram showing branches of descent from a common ancestor. Haplogroups and their evolution can be mapped as a phylogenetic tree.

Segment

A shared segment is a stretch of identical chromosome shared between you and a match. The longer the unbroken segment the more closely you tend to be related to your match.

SNP (single nucleotide polymorphism)

SNPs are pronounced ‘snips’. A SNP is a variation in a single nucleotide. When our genes recombine changing occurs in the coding. This variation is called a SNP. As mentioned above, a shared segment is a stretch of identical chromosome. Each end of the segment is determined by a SNP, and the distance between the two SNPs is referred to in centiMorgans.

Subclade

A supgroup of a haplogroup.

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