While we are waiting for FTDNA to go live with X chromosome segment (SNP) matching, best to discuss more fun facts.

People often ask where to start when they are matched with a cousin on the X chromosome and you have no idea how you are related. The goal is to find a most recent common ancestor (MRCA) with that cousin that falls within the X pattern of inheritance. So best to look at math probabilities first.

As the percentage of ancestral X chromosome participants decreases as you go back in time, what happens to the probabilities? Do all the X participants 7 generations back in your pedigree have an equal chance of giving you an X segment? Absolutely not. Some have a much, much greater chance than others even though they all fall within the X pattern of inheritance in the same generation. Did you know that you have some favored X ancestors in each generation? The more males in the path from the ancestor down to you, the higher the chance of getting an X segment from that line. This is because the segments do not get cut up through recombination.

That doesn't mean you will get an X from these males. Some people like my mother gave me her mother's X and for some reason she hardly mixed it in with her father's X at all so that entire X lineage was lost to me in one fell swoop. I have to depend on my siblings to see the segments coming from my maternal grandfather since my parents are deceased. So much for probability...

Well, lets get back to the math probabilities anyway even if you can't depend on math in real life to do what you want it to do sometimes. For many people, the probabilities can often lead you in the right direction so you don't waste as much time.

http://www.thegeneticgenealogist.com.../NewChart5.jpg shows some probabilities. One fifth great-grandmother has a 1 in 8 chance but another male and female in that same generation only have a 1 in 64 chance each of making contributions to your X. The difference becomes even greater as you go back in time. The 5th great-grandmother who gave you her entire mitochondrial DNA really doesn't have the same advantage when it comes to being a participant in your X chromosome contribution. Remember that the mitochondrion and the X chromosome are inherited entirely separately. There is a very high chance the X will be broken up and/or lost when it passes through several females. So the mitochondrial all-female path becomes less and less important for the X chromosome as you go back in time. The lines that show male-female-male-female etc. have a higher success rate for X chromosome survival as a general rule.

The X chromosome pedigree can be very helpful in knowing where to start. You want to find a MRCA (or ancestors) when you have an X matching segment with a newly-identified cousin. Check the probabilities first and concentrate on those lines to the left in Blaine Bettinger's fan chart where the fathers are listed to the left of their female mates.

Lean to the left...

Or if you want to follow a horizontal pedigree chart with the males listed on top, aim for the line on top as you read left to right following the X-inheritance pathway. See Deb's Delvings here:

http://debsdelvings.blogspot.com/201...ce-charts.html

This is just a suggested approach and you may find it impossible to connect with a distant cousin's pedigree. There is a reasonable chance your connection is with that 10th great-grandparent who had an amplified 1 in 64 probability of giving you a contribution. If this cousin also has an autosomal match with you, your chances are higher that there is a much more recent MRCA. Nobody can really make a prediction based on the size of the segment on just one X chromosome in isolation.

Kathy

People often ask where to start when they are matched with a cousin on the X chromosome and you have no idea how you are related. The goal is to find a most recent common ancestor (MRCA) with that cousin that falls within the X pattern of inheritance. So best to look at math probabilities first.

As the percentage of ancestral X chromosome participants decreases as you go back in time, what happens to the probabilities? Do all the X participants 7 generations back in your pedigree have an equal chance of giving you an X segment? Absolutely not. Some have a much, much greater chance than others even though they all fall within the X pattern of inheritance in the same generation. Did you know that you have some favored X ancestors in each generation? The more males in the path from the ancestor down to you, the higher the chance of getting an X segment from that line. This is because the segments do not get cut up through recombination.

That doesn't mean you will get an X from these males. Some people like my mother gave me her mother's X and for some reason she hardly mixed it in with her father's X at all so that entire X lineage was lost to me in one fell swoop. I have to depend on my siblings to see the segments coming from my maternal grandfather since my parents are deceased. So much for probability...

Well, lets get back to the math probabilities anyway even if you can't depend on math in real life to do what you want it to do sometimes. For many people, the probabilities can often lead you in the right direction so you don't waste as much time.

http://www.thegeneticgenealogist.com.../NewChart5.jpg shows some probabilities. One fifth great-grandmother has a 1 in 8 chance but another male and female in that same generation only have a 1 in 64 chance each of making contributions to your X. The difference becomes even greater as you go back in time. The 5th great-grandmother who gave you her entire mitochondrial DNA really doesn't have the same advantage when it comes to being a participant in your X chromosome contribution. Remember that the mitochondrion and the X chromosome are inherited entirely separately. There is a very high chance the X will be broken up and/or lost when it passes through several females. So the mitochondrial all-female path becomes less and less important for the X chromosome as you go back in time. The lines that show male-female-male-female etc. have a higher success rate for X chromosome survival as a general rule.

The X chromosome pedigree can be very helpful in knowing where to start. You want to find a MRCA (or ancestors) when you have an X matching segment with a newly-identified cousin. Check the probabilities first and concentrate on those lines to the left in Blaine Bettinger's fan chart where the fathers are listed to the left of their female mates.

Lean to the left...

Or if you want to follow a horizontal pedigree chart with the males listed on top, aim for the line on top as you read left to right following the X-inheritance pathway. See Deb's Delvings here:

http://debsdelvings.blogspot.com/201...ce-charts.html

This is just a suggested approach and you may find it impossible to connect with a distant cousin's pedigree. There is a reasonable chance your connection is with that 10th great-grandparent who had an amplified 1 in 64 probability of giving you a contribution. If this cousin also has an autosomal match with you, your chances are higher that there is a much more recent MRCA. Nobody can really make a prediction based on the size of the segment on just one X chromosome in isolation.

Kathy

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