Chromosome Browsers for Genealogy: What Are They and Why Are They Useful?
A helpful guide detailing how to use chromosome browsers in your genealogy research
Chromosome browsers are tools that allow you to see the unique DNA segments, or blocks of DNA on chromosomes, shared between you and either one genetic match or a set of genetic matches. It is typically displayed showing your 23 chromosomes with colors indicating shared segments. Using a chromosome browser, you can view the start and stop locations of specific shared segments.
How are Chromosome Browsers a useful tool for genealogy?
Genetic genealogy, or DNA analysis of your DNA test results, is one tool in extending your family trees. It is most effective in breaking down your brick walls, or dead ends, when you can go no further. Your genetic match lists are the most effective part of that genetic genealogy, and chromosome browsers allow you to view how you share DNA with each of your genetic matches, providing more information than just simply sharing DNA. Do you share segments of DNA with multiple matches? Are the segments of DNA immediately adjacent to each other? How the DNA is shared between you and your matches can provide more clues that could help you extend your trees and break through your genealogical brick walls.
What are you seeing on the Chromosome Browser?
You have 23 pairs of chromosomes: one set of 23 chromosomes inherited intact from your mother and one set of 23 chromosomes inherited intact from your father. While the chromosomes reside tightly twisted in the cell’s nucleus at a microscopic level, if you were to lay them straight against each other, each pair (one chromosome from your mother and one from your father) could be arranged from the longest to shortest and numbered 1-23. Each chromosome is uniquely identified by number. Because the last pair of chromosomes are either XX or XY, the sex chromosomes, some chromosome browsers only highlight the autosomal pairs, or chromosomes 1-22. Additionally, some chromosome browsers show two chromosomes for each set and one shows one lines to represent both in the set.
Chromosome browsers show where your genetic match is sharing DNA with you. If you consider comparing a parent and a child in a chromosome browser, the image will show a single continuous color on one of each of the chromosome pairs. That is because the parent matches the child on one of the two chromosomes from each pair and in many browsers, this appears as a complete match on all chromosomes, when in reality it is only a 50% match.
Parents and identical twins are the only matches that will show continuous shared DNA segments on all chromosomes. Beyond these relationship levels, the amount of DNA shared in common varies due to a random process called recombination which shuffles the DNA each generation and breaks up segments of shared DNA. Using a mother’s DNA as an example, in recombination the mother’s DNA, inherited from her parents, intertwines and separates in a few places before being passed on to the child. The DNA segment from mother to child may have alternating maternal grandfather and maternal grandmother segments. Likewise, the segments inherited from these maternal grandparents were in turn inherited from some of the maternal grandparents’ ancestors. It is important to note that this recombination does not always happen, and a chromosome can be passed intact from one grandparent to a parent and then down to the child. Sometimes entire chromosomes can be inherited from more distant ancestors as well.
If you compare maternal first cousins in a chromosome browser, only their shared segments, identical from beginning to end, are displayed. Perhaps you inherited a longer piece from one grandparent than your first cousin did, yet the colored region is only where the DNA is identical, or shared. Similarly, your second cousins will have inherited some segments in common from each of your shared great-grandparents, some of which are uniquely inherited only by them and some of which are inherited in common with you and other genetic cousins.
How is this information genealogically useful?
Each of your segments come from one of your ancestors. By comparing shared segments of genetic matches through chromosome browsers, you can make conclusions about your genetic matches and, therefore, your ancestry. Some of those conclusions can even lead to a breakthrough in your genealogical brick walls.
- Segment Length: Longer segments, often combined with additional smaller segments, indicate a more recent relationship between you and your genetic matches. Many short segments, when the estimated relationship is expected to be a closer match, may mean the shared DNA between you and your match comes from an endogamous community or multiple distant common ancestors. Perhaps you have Jewish ethnicity and you are trying to determine which of your fourth to sixth cousin matches are actually the closest related. Examine the segment lengths and those with larger segments are closer related than others with only small segments.
- Matches descended from other matches: We frequently encourage family members to test and so it is very common to find descendants of matches within a match list. If one match only shows same sized or smaller sized matching segments than the other match in the comparison, and no segments outside of the shared segments, it is possible that the first match descends from the other match. If you determine two genetic matches are likely father/son, you can then focus only on the father’s shared DNA, as the son only inherited half of his father’s DNA and is not as genetically useful. If you are attempting to reach the father to collaborate and ask about family stories, you can reach out to the son in hopes that he can connect you with his father.
- Triangulated Segments: If you and two matches share an identical segment (and a common ancestor), then you have a triangulated segment. There are several online tools that assist in this triangulation analysis. Each company’s chromosome browser may display the triangulated segments differently. For instance, if you have selected multiple people to compare, it may be that only the triangulated segments across all compared matches will be shown. Be systematic about your comparisons to ensure no triangulated segments are overlooked. Identify the start and stop locations for the shared segment between all the members of the triangulated matches. Use triangulated segments to determine how your genetic matches are related to you. Perhaps you are wondering if a genetic match is a maternal or paternal match or on which line they match you. Compare their DNA segments in common with you to other matches and if they share with others in the same place, you can determine from which line they descend.
- Triangulated Groups: Those with whom you share triangulated segments can be placed in a triangulated group and together you can work to determine if your overlapping segments can be traced to a specific common ancestor. It is most helpful to have genetic matches in this triangulated group at each step of the way to the common ancestor. For instance, if you have a 3xgreat-grandparent, attempt to include a cousin, second cousin, third cousin, and fourth cousin from each step in your direct line. This will help to confirm the validity of your triangulated group and identify with certainty your common ancestor.
- Chromosome Mapping: With enough shared segments among different close cousins, you can assign specific segments to known ancestors. To know which segments are inherited through your maternal grandfather versus which segments are inherited through your maternal grandmother, you would group and triangulate the segments with close cousins and collateral relatives of those individuals comparing shared segments. With enough shared segments among different close cousins, you may even be able to identify the crossover points for your ancestral segments. Your inheritance can be mapped on a chromosome map, showing from which ancestors your segments of DNA were inherited. This map assists you when comparing new and unknown genetic cousins to determine their likely relationships. If they consistently match in the same regions previously assigned to a particular ancestor, you can hypothesize that they also descend from that mapped ancestor or an ancestor of that mapped ancestor. Many tools can assist with chromosome mapping.
- Overlapping Opposite Segments: If, when comparing two matches separately in a chromosome browser, they each match you at the same portion of a chromosome but do not share that same portion with each other, this is because one is related through your maternal ancestry and one is related through your paternal ancestry. If you think two matches are related through your mother and yet they have overlapping opposite segments, you can then know that you inherited DNA in common with one match through your mother and you inherited DNA in common with the other match through your father. This does not mean that the two matches aren’t also in some way related, as is common in intermarrying communities.
- Immediately adjacent segments: If a genetic match on the chromosome browser shows a segment that stops immediately adjacent to another match’s start of a new segment, it is likely that they are related on the same side of your family (either both maternal or both paternal), possibly even diverging at your most recent ancestral couple, with one as the relative of the wife and the other as the relative of the husband. This is because as recombination occurs, the break in the chromosomes after they exchange information rotates between the maternal and paternal chromosome. We call this crossover point a recombination point. If you have one match who is a known match through your father’s line and another match with an unknown relationship, and the two show immediately adjacent segments in comparison to your DNA, it is likely both are related through your paternal side.
The best way to understand the chromosome browser is to begin using it. Start with any known relationships between you and those in your genetic match list and become familiar with appropriate segment length for known relationships, identify your immediately adjacent segments, triangulated segments, and form triangulated groups of matches. Perhaps you will enjoy your chromosome puzzle and will decide to map your chromosomes to your furthest genetically confirmed ancestors.
If you’ve taken a DNA test and need help analyzing the results, or if you have a genealogy question you think DNA might be able to help answer, we would love to help! Contact us to discuss your questions and goals, and we’ll help you choose a project option and get started.