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Genetic Distances

When trying to reconstruct a deep family tree, some measure (or metric) of genetic distance is required. In an ideal world, we could directly count the total number of mutations at which two chromosomes differ. This is not be quite as straightforward as it sounds, as it makes assumptions about the underlying mutation process. For most y-chromosome markers, the standard model is the Stepwise Mutation Model. This follows the changes in length, allowing a step up (or down) by one at each mutation. Note that this means that an observed match could result from no mutations, from one up and one down mutation, from two up and two down mutations, etc. For a genetic distance, we wish to translate the observed number of differences in a marker allele between two individuals (say an allele of length 24 and one of length 22) into the actual number of mutations that have occurred. Under the most simple stepwise mutational model, mutations are equally likely to increase or decrease your allelic state by one step, and [our] calculations assume this symmetric single step model.                                                   -- Family Tree DNA

Recently, this method of counting genetic distance has been called into question, as a result of which, FTDNA has partially modified the method they use. Their current thinking may be found by clicking <Understanding Genetic Distance.> [link no longer active] In addition, there has been significant discussion of the effect of recombinational loss of heterozygosity (recLOH) on the measurement of genetic distance. A summary of this discussion may be found here: recLOH as well as this Project's recommended Non-recLOH Event Multi-copy Marker Distance Counting Protocol.

For Combined Marker Participant Distance Table, click .

For Combined Time to Most Recent Common Ancestor Table, in years, click .

For an excellant table of allele frequencies by haplotype.

Go here to read about the Sorenson Molecular Genealogy Foundation.

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Understanding Your Results: Matching Other Surnames

For Y DNA test results, matches with other Surnames can occur. These matches are the result of one of the following events:

1. You share a common ancestor before the establishment of surnames
2. Convergence: where both participant's result mutated and now match
3. An adoption
4. An extramarital event
5. A branch of the family adopted a different surname

Matches with other surnames are typically more prevalent with those who are Haplogroup R1b.

Most likely, when you match some one with a different surname, you share a common ancestor before the estab-
lishment of surnames or convergence occurred.

Imagine a situation 3 generations before surnames, where our imaginary ancestor "Rad" had 5 sons from his first wife, who then died, and 2 sons from his second wife. In the next generation, these 7 sons had a total of 27 sons who lived to adulthood. These 27 sons then had 108 sons.
Each of these 108 sons had Rad's Y DNA result. Over time, many of Rad's sons moved away to other villages, often when they married, and sometimes they were looking for a better situation. Many of Rad's grandsons then moved away from their father, maybe only to the next village. The 108 males in this family tree are now spread out geographically. A few had even traveled a long way from their ancestral homeland looking for a better situation.
As surnames became established, it is very possible that most of Rad's grandsons ended up with different surnames.
Rad's descendents are not the only ones with the same Y DNA result. Rad also had 3 brothers, who had descendents. Rad's father had 4 brothers, who also had descendents. Rad's grandfather also had brothers who had descendents. There were many males who had Rad's Y DNA result, or a close result if there had been a mutation. Each of these males could have adopted a different surname. At the minimum, those in different places most likely would have taken on a different surname. Also, those who did not know that they were distantly related probably took on a different surname.
Today, there would be many males with Rad's Y DNA result - and a wide variety of surnames, spread over an even larger geographical area. Since these matches are before the adoption of surnames, they are not worth pursuing for your family tree.

The value of these matches is that they could provide clues regarding the ancestral location, for those that have not been able to discover the ancestral location.

Another event that can result in Y DNA results matching for males with a different surname is called convergence. Convergence is a scientific term that applies when two Y DNA results have mutated so that they now match each other. Convergence is explained in detail in the following issue of our newsletter: Haplotypes: Convergence

Adoptions occurred in the past, although they weren't formal procedures like today. A widow could remarry, and the children took on the surname of the new husband. A child could be abandoned, and a family took in the child, and the child assumed the surname of the family.

Before pursuing Y DNA matches with another surname based on assuming that an adoption occurred, first review your family history research to determine if there is any evidence to support a possible adoption. For example, do you have a widow remarrying and the new husbands surname matches one of those surnames of the DNA results that you match? Do you have a child in your direct male line who appears in a census, yet you can't find the birth record? Are any of the surnames your DNA result matches found in the locations where your ancestors lived? Have any children disappeared between censuses, and you do not find a death record?

If you don't have any evidence of an adoption in your family tree, then it probably isn't worth pursuing a Y DNA match with another surname under the assumption that there is an adoption.

Extramarital events occur, including illegitimate births. Extramarital events where the female is married will be the most difficult to track down. For an illegitimate birth, typically the Parish Registers will note that the person being baptised is illegitimate, and only rarely does a Parish Register indicate the father. Often even the death of the person will indicate that they are illegitimate, since illegitimacy carried such a stigma for the person's whole life.

From your family history research, you would most likely know if your direct male Line includes an illegitimate birth. You have probably also validated the Y DNA result for your family tree, so you would have identified a problem if the two results didn't match, and most likely have undertaken additional research and done additional testing to resolve the situation with your family tree. If you wonder if an illegitimate birth occurred further back in your family tree, then your best course of action is to pursue research to take your family tree back further, before pursuing matches with other surnames.

Extramarital events where the female is married are much more difficult to track down. There must be some evidence to make this conclusion. For example, did the descendents of the first son match others with the family surname, and descendents of the last son don't match the surname result? Was there a later divorce and remarriage? If so, do the surname of the second husband match any of those surnames for the Y DNA match?

Another event that can result in Y DNA matching others with a different surname is when a branch of the family tree takes on a different surname. There are many reasons why a surname could be changed. Perhaps, it is simply personal preference, or the family immigrated to a new country and wanted to fit in. A husband could take on the wife's surname, to prevent her surname from becoming extinct in her family tree. The surname could also have evolved into a different form when migration is combined with illiteracy. The person migrating could only say their surname, and the spelling could be dramatically different in a new location with a different language or accent.

Most likely you would have some clues in your family history research as to whether a different surname is possible. Do you have a missing person of family group? If all the people are accounted for, then most likely, assuming a different surname by a branch of your tree is not the reason that you have a Y DNA match with a different surname.

Most likely, Y DNA matches with other surnames are a result of being related through a common ancestor prior to surnames, or through convergence.

A match with another person is always exciting. The question then becomes, do you pursue the match? The first step before pursuing a match is to upgrade your test to 37 Markers, to see if the match still occurs. In most cases, there will no longer be a match. The next step would be to review your family history research to determine if there is possibly an adoption, surname change, or extramarital event. If you don't find any clues to support the possibility of these events, then it is reasonable to assume that the Y DNA match came from a common ancestor prior to surnames or convergence.

Those who are Haplogroup R1b will tend to have DNA matches with other surnames. Haplogroup R1b is the largest population group in Europe, and therefore due to the size and scope of this large population, there have been many opportunities for convergence.

If you belong to a Surname Project, you can eliminate seeing Y DNA Matches with other surnames by setting your Public/Private setting to Private. [Not recommended by this Administrator.] The Public/Private setting determines whether the search for Y DNA matches will only look for matches within the Surname Project or will look for matches in the whole Family Tree DNA customer database of those set to Public.
                                                                                                                    - Family Tree DNA


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Last Revised February 10, 2008

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