DNA

[Paul]

Hello all:

I have a DNA question I hope someone with more knowledge can answer. Myself and a few other family members tested with 23andMe. My oldest sister (who is actually my half sister (different fathers)) and I recently discovered we have a half brother. You guessed it, we all have different fathers. While looking at mine and Dean’s (half brothers given name) shared segments, how can we have completely identical match on the X chromosome when we have different fathers? My paternal haplogroup is R-P311 while Dean’s is R-CTS6. All three of us share the same maternal haplogroup U5b.......

Paul G.

[Cheri Mello]

Chromosome 23 is your sex chromosome that determines if you are a man or a woman. A man inherits the Y from his dad and an X (in tact) from his mom. A woman inherits 2 Xs. So this is why you are seeing a full X on the 23rd chromosome.

Cheri

Cheri Mello
Listowner, Azores-Gen
Researching: São Miguel island: Vila Franca, Ponta Garca, Ribeira Quente, Ribeira das Tainhas, Achada

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[Ângela Loura]

You receive XX from your mother and YX from your father. 

[Paul]

My bad. I (obviously) was thinking the other way.

[Tomas Leal]

The XX configuration is tricky, which many either forget or do not realize. The Y-gene is so easy to follow because it always comes directly from father to son, father to son, at so on. My Y-gene is the exact same one my several times great-grandfather had (unless some genetic mutation occurred in between).

With a female child, each parent contributes an X gene. Which grandmother that particular X gene came from is another matter.

The X gene passed father to daughter had to come from the father's mother; however, she had an X gene from each parent, so the X gene the father passed on could have come either from his mother's mother or his mother's father. In turn, that X gene could have come from different grandmothers.

What this means is full sisters can have different X genes via their mother, as the one from their mother passed could have come from any one of three grandmothers (either of two passed from the mother's parents or  one passed by the father)! Add in great-grandmothers and it's even more complicated.

In the case presented here, the fathers were different so the X gene each one passed could be from a different grandmother as well. Thus, we can see the importance of "triangulation" of getting DNA samples from first cousins for comparisons, in addition to that from siblings.

Tomás Leal

[MaryAnn Santos]

Thank you for this explanation. It is very helpful.

Best,

MaryAnn

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MaryAnn Santos

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[Cheri Mello]

Here is Roberta Estes' "X Marks the Spot" article (with pictures) from her "DNA Explained" blog:

https://dna-explained.com/2012/09/27/x-marks-the-spot/

Cheri Mello
Listowner, Azores-Gen
Researching: São Miguel island: Vila Franca, Ponta Garca, Ribeira Quente, Ribeira das Tainhas, Achada

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[Liz Migliori]

Thank you  this is the best and easiest to understand that I’ve heard yet

Sent from my iPhone

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[MaryAnn Santos]

This is great, thank you, Cheri!

MaryAnn

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