X Chromosome - completely identical??

[Liz_in_Sussex]

Hi,

I am looking at the Genome View in 23andme and have noticed that 3 people all match each other and my Dad on a section of the X chromosome and it is coloured dark blue to indicate 'completely identical'.  I am trying to get my head round what this means.

I think I am right in saying that Dad got his X from his mother?  If so, presumably these other 3 men are all descended from this branch of our tree too?

But this is where I get muddled.  Could they be descended from either my Granny's mother OR father - not necessarily her mother, as she got an X from each of them?

I hope that makes sense.  I have attached a screenshot of a bit of the chart to show the little match - it puzzles me as to why my match with Dad is only 'half identical' but these three men are all 'completely identical'.    But there is no question that I am his daughter 

Thanks,

Liz

[Erato]

Keep in mind that chromosomes are not passed intact from one generation to the next.  The process called 'crossing over' ensures that the genetic material is mixed and recombined during meiosis.  Consequently, the X chromosome that you got from your mother contains bits from each of the two X chromosomes she had received from her parents.

https://en.wikipedia.org/wiki/Chromosomal_crossover

[Liz_in_Sussex]

  So my Dad received an X chromosome from his mother but it was a mixture of X from HER parents ... in which case we have no way of narrowing down which side of the family these people are descended from that all match him at that spot on the X chromosome?

Or are you saying that the reason mine is only 'half identical' is because mine is mixed with what I have inherited form my mother?

Sorry  but thanks for replying.

Liz

[hurworth]

Your father should be showing as half-identical to you along the entire X-chromosome, just like he is half-identical on each other chromosome. 

I think he shows as fully identical on this segment with other men because he has only one X chromosome and so do they.  You can't be half-identical when each of you have just one X-chromosome.  You match or you don't.

These men and your father all received their X-chromosome from their mothers.   It is still possible for matching segments on other chromosomes are from their fathers though.

I have seen instances where a grandson is a full X-match with his maternal grandfather, so it doesn't always get mixed.

[Liz_in_Sussex]

Hi,

Thank you for your reply.  Yes, Dad does show as half identical to me along the whole X chromosome.

I think he shows as fully identical on this segment with other men because he has only one X chromosome and so do they.  You can't be half-identical when each of you have just one X-chromosome.  You match or you don't.

- That makes sense ... so (and I apologise if this is really obvious!) does that mean Dad matches these men somewhere on his mother's side of the family, but we wouldn't be able to tell if it is his maternal Grandmother's or Grandfather's line?

He does have matching segments on other chromosomes with people who we know are related to him through his father.  Unfortunately he doesn't have enough (known) cousins to be able to triangulate results for each branch of the family.

Thanks for your help.

Liz