PDA

View Full Version : Grandfather's X chromossome Inheritance



Kaipiro
11-08-2018, 01:26 PM
So, this image is my X and both my grandmother's X compared we seem to share only the tip of our Xs, by proportions it seems like >10% <20% of it, so I basically have my maternal grandfather X chromo. This is so small in fact that a random woman with no autossomal matches with my grandma shares more X with her than me, her own grandson. is this normal? or under a "normality"?

I'm only concerned to the fact that my grandfather is going bald, but I looked up the genes I don't think I have the balding gene there, would I have escaped with the tiny piece from my grandma? luck?

https://i.imgur.com/OI4VjGx.png

geebee
11-08-2018, 05:16 PM
So, this image is my X and both my grandmother's X compared we seem to share only the tip of our Xs, by proportions it seems like >10% <20% of it, so I basically have my maternal grandfather X chromo. This is so small in fact that a random woman with no autossomal matches with my grandma shares more X with her than me, her own grandson. is this normal? or under a "normality"?

I'm only concerned to the fact that my grandfather is going bald, but I looked up the genes I don't think I have the balding gene there, would I have escaped with the tiny piece from my grandma? luck?

https://i.imgur.com/OI4VjGx.png

Since you recognize that the DNA on your X chromosome that you don't share with your maternal grandmother must have come from your maternal grandfather, I don't understand why you seem amazed that "a random woman with no autossomal matches with my grandma shares more X with her than me, her own grandson". You simply share this DNA with your maternal grandfather and his mother instead. Why would your grandmother share autosomal DNA with either her husband or her mother-in-law? (Yes, they could be related -- but I presume you'd know it if they were.)

What you're seeing is nothing unusual. In fact, it's possible for a male to share no X-chromosome DNA with his maternal grandmother.

The graphic below shows how my oldest brother Bernie, our sister Kim, and I match on the X chromosome.

26963
26964

As you can see, Kim and Bernie match across the entire X chromosome except for the very tips. This actually doesn't mean they don't match at the tips, but simply that FTDNA doesn't believe there are enough tested SNPs here to be able to tell. (23andMe does show these siblings matching at the tips of the X chromosome.)

Based on the matching of my siblings to various cousins, it's possible to say that this X came entirely from our mother's father, and therefore from our mother's paternal grandmother.

That means that my brother Bernie doesn't share anything at all on the X chromosome with our maternal grandmother. Nor does our sister Kim. And I share less with my maternal grandmother than you do with yours -- only about 20 cM.

As you can see from the graphic, there's only a small region in which I don't match these two siblings. As I said, the regions where we do match came from our mother's father, so the region where we don't came from our mother's mother in my case.

I know it's from my maternal grandmother because I match some maternal half 2nd cousins across this region. I also know it came from my grandmother's mother, because my matches here are all descendants of my grandmother's maternal half sisters.

Kaipiro
11-08-2018, 06:33 PM
Since you recognize that the DNA on your X chromosome that you don't share with your maternal grandmother must have come from your maternal grandfather, I don't understand why you seem amazed that "a random woman with no autossomal matches with my grandma shares more X with her than me, her own grandson". You simply share this DNA with your maternal grandfather and his mother instead. Why would your grandmother share autosomal DNA with either her husband or her mother-in-law? (Yes, they could be related -- but I presume you'd know it if they were.)

What you're seeing is nothing unusual. In fact, it's possible for a male to share no X-chromosome DNA with his maternal grandmother.

The graphic below shows how my oldest brother Bernie, our sister Kim, and I match on the X chromosome.

26963
26964

As you can see, Kim and Bernie match across the entire X chromosome except for the very tips. This actually doesn't mean they don't match at the tips, but simply that FTDNA doesn't believe there are enough tested SNPs here to be able to tell. (23andMe does show these siblings matching at the tips of the X chromosome.)

Based on the matching of my siblings to various cousins, it's possible to say that this X came entirely from our mother's father, and therefore from our mother's paternal grandmother.

That means that my brother Bernie doesn't share anything at all on the X chromosome with our maternal grandmother. Nor does our sister Kim. And I share less with my maternal grandmother than you do with yours -- only about 20 cM.

As you can see from the graphic, there's only a small region in which I don't match these two siblings. As I said, the regions where we do match came from our mother's father, so the region where we don't came from our mother's mother in my case.

I know it's from my maternal grandmother because I match some maternal half 2nd cousins across this region. I also know it came from my grandmother's mother, because my matches here are all descendants of my grandmother's maternal half sisters.

interesting case, thanks for sharing. I am amazed cause everyone else seems to have a good chunk of their X from their maternal grandmother (friends that tested). It is also to prove that those X chrome inheritance charts are flawed and are not "precise math", they are simply illustrative.

Saetro
12-08-2018, 07:31 PM
It is also to prove that those X chrome inheritance charts are flawed and are not "precise math", they are simply illustrative.
The charts that show you CANNOT inherit X from certain ancestors are accurate. You can rely on them.
Every chart I have come across with percentages has had a statement attached that these averages seldom apply exactly.
The good ones add that X transmission from one generation to the next - where it IS permitted - can be anything from nothing at all to 100%.

Like everything else in DNA genealogy, read the instructions first.
If there are no instructions, find some: something that describes what is going on.
It saves money, time, effort; although lacks excitement - and frustration.
(And something in me loves a roller coaster, so I do not always follow the above - as long as the consequences won't be too dire.)

JerryS.
12-08-2018, 08:31 PM
my great-grandpa passed his X on to my grandma. my grandma passed her X to my dad. my dad passed his Y to me. I have no direct line to my great-grandpa is that correct?

Kaipiro
12-08-2018, 09:52 PM
my great-grandpa passed his X on to my grandma. my grandma passed her X to my dad. my dad passed his Y to me. I have no direct line to my great-grandpa is that correct?
only the X chromosome (which came from his mother and you got Y from your father's father), you are a direct descendant of your great-grandfather and have parts of his autossomal DNA.