---
title: "Genetic genealogy"
chunk: 3/3
source: "https://en.wikipedia.org/wiki/Genetic_genealogy"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T04:14:35.876877+00:00"
instance: "kb-cron"
---

=== Pedigree family trees ===
Pedigree family trees have traditionally been prepared from recollections of individuals about their parents and grandparents.  These family trees may be extended if recollections of earlier generations were preserved through oral tradition or written documents.  Some genealogists regard oral tradition as myths unless confirmed with written documentation like birth certificates, marriage certificates, census reports, headstones, or notes in family bibles.  Few written records are kept by illiterate populations, and many documents have been destroyed by warfare or natural disasters.  DNA comparison may offer an alternative means of confirming family relationships of biological parents, but may be confused by adoption or when a mother conceals the identity of the father of her child.
While mitochondrial and Y-chromosome DNA matching offer the most definitive confirmation of ancestral relationships, the information from a tested individual is relevant to a decreasing fraction of their ancestors from earlier generations.  Potential ambiguity must be considered when seeking confirmation from comparison of autosomal DNA. The first source of ambiguity arises from the underlying similarity of every individual's DNA sequence.  Many short gene segments will be identical by coincidental recombination (Identical by State: IBS) rather than inheritance from a single ancestor (Identical by Descent: IBD).  Segments of greater length offer increased confidence of a shared ancestor.  A second source of ambiguity results from the random distribution of genes to each child of a parent.  Only identical twins inherit exactly the same gene segments.  Although a child inherits exactly half of their DNA from each parent, the percentage inherited from any given ancestor in an earlier generation (with the exception of X chromosome DNA) varies within a normal distribution around a median value of 100% divided by the number of ancestors in that generation.  An individual comparing autosomal DNA with ancestors of successively earlier generations will encounter an increasing number of ancestors from whom they inherited no DNA segments of significant length.  Since individuals inherit only a small portion of their DNA from each of their great-grandparents, cousins descended from the same ancestor may not inherit the same DNA segments from that ancestor.  All descendants of the same parent or grandparent, and nearly all descendants of the same great-grandparent, will share gene segments of significant length; but approximately 10% of 3rd cousins, 55% of 4th cousins, 85% of 5th cousins, and more than 95% of more distant cousins will share no gene segments of significant length.  Failure to share a gene segment of significant length does not disprove the shared ancestry of a distant cousin.
The best autosomal DNA method for confirming ancestry is to compare DNA with known relatives.  A more complicated task is using a DNA database to identify previously unknown individuals who share DNA with the individual of interest; and then attempting to find shared ancestors with those individuals.  The first problem with the latter procedure involves the relatively poor family history knowledge of most database populations.  A significant percentage of individuals in many DNA databases have done DNA testing because they are uncertain of their parentage, and many who confidently identify their parents are unable or unwilling to share information about earlier generations.  It may be easier to identify a shared ancestor in the fortunate situation of shared DNA between two individuals with comprehensive family trees, but finding multiple shared ancestors raises the question of from which of those ancestors was the shared segment inherited.  Resolving that ambiguity typically requires finding a third individual sharing both the ancestor and the gene segment of interest.

=== Ancestral origins ===

A common component of many autosomal tests is a prediction of biogeographical origin, often called ethnicity. A company offering the test uses computer algorithms and calculations to make a prediction of what percentage of an individual's DNA comes from particular ancestral groups. A typical number of populations is at least 20. Despite this aspect of the tests being heavily promoted and advertised, many genetic genealogists have warned consumers that the results may be inaccurate, and at best are only approximate.
Modern DNA sequencing has identified various ancestral components in contemporary populations. A number of these genetic elements have West Eurasian origins. They include the following ancestral components, with their geographical hubs and main associated populations:

=== Human migration ===

Genealogical DNA testing methods have been used on a longer time scale to trace human migratory patterns. For example, they determined when the first humans came to North America and what path they followed.
For several years, researchers and laboratories from around the world sampled indigenous populations from around the globe in an effort to map historical human migration patterns. The National Geographic Society's Genographic Project aims to map historical human migration patterns by collecting and analyzing DNA samples from over 100,000 people across five continents. The DNA Clans Genetic Ancestry Analysis measures a person's precise genetic connections to indigenous ethnic groups from around the world.

=== Law enforcement ===

Law enforcement may use genetic genealogy to track down perpetrators of violent crimes such as murder or sexual assault and they may also use it to identify deceased individuals. Initially genetic genealogy sites GEDmatch and Family Tree DNA allowed their databases to be used by law enforcement and DNA technology companies  to do DNA testing for violent criminal cases and genetic genealogy research at the request of law enforcement. This investigative, or forensic, genetic genealogy technique became popular after the arrest of the alleged Golden State Killer in 2018, but has received significant backlash from privacy experts. However, in May 2019 GEDmatch made their privacy rules more restrictive, thereby reducing the incentive for law enforcement agencies to use their site. Other sites such as Ancestry.com, 23andMe and MyHeritage have data policies that say that they would not allow their customer data to be used for crime solving without a warrant from law enforcement as they believed it violated users' privacy.

== See also ==

== References ==

== Further reading ==

=== Books ===

=== Documentaries ===

=== Journals ===

== External links ==

Shared cM Project – how to determine ones relationship based on Centimorgan (cM) values