Thursday, January 15, 2009

Our X chromosome, pt. 2

Following is the second of the columns by Blaine Bettinger, PhD. from his excellent blog The Genetic Genealogist which I am again going to copy verbatim here.

This is again entirely Dr. Bettinger's work as published by him on Jan. 12, 2009.

More X-Chromosome Charts

Last month I wrote “Unlocking the Genealogical Secrets of the X Chromosome” and posted a few charts that show the inheritance of the X-chromosome through 8 generations.    I thought these charts might be helpful since inheritance of the X-chromosome can be difficult to understand without seeing it.

New Chart with Ahnentafel Numbers

Since posting the article, two new charts have been created using the originals.    I made one, and the other was made by Rodney Jewett (who gave me permission to re-post the chart here) and posted at

Mr. Jewett added the Ahnentafel numbers of contributing X-chromosome ancestors to the chart. Using these numbers, an individual can simply create a numbered Ahnentafel report to identify X-chromosome contributing ancestors using this chart:

Note that Ann Turner also has a text file of the Ahnentafel numbers of those ancestors who potentially contributed to the X chromosome, through 10 generations.

New Chart Showing Contribution Percentages

I’ve been very intrigued by all the recent discussion of the X-chromosome in the genetic genealogy context, and I’ve been exposed to some facts about the X-chromosome of which I previously was unaware.

For instance, I kept seeing statements that suggested that different ancestors in each generation contributed different amounts to the X chromosome.    I read that at the 8th generation, one ancestor contributes 1/8th, some contribute 1/64th, and the others contribute between 1/16th and 1/32nd.    I kept trying to understand this, but I had to chart it out to grasp it.    Although this chart is for a male, I believe it will also work for a female by adjusting the generations slightly.    The chart is big so it might take a moment to download:

Thus, the chart shows that if you are a male, your mother’s father’s mother’s father’s mother’s father’s mother contributed 1/8th of your X-chromosome, while your mother’s mother’s mother’s mother’s mother’s mother’s parents only contributed 1/64th.

Now, keep in mind that this chart serves only as a very rough guideline for inheritance.    Although males pass the X chromosome largely unchanged to their daughters, females will usually pass a ‘mixed’ X chromosome to their child   –   a mixture of the X chromosome they received from their father and the X chromosome they received from their mother.    However, a child is unlikely to receive an X chromosome from their mother that is 50% from their maternal grandfather and 50% from their maternal grandmother   –   it will most likely be some other more random amount between 0% and 100%.    Thus, an ancestor is likely to be either under- or over-represented in an actual X chromosome.

Please, if you find any errors in my chart, please feel free to email me and I’ll do my best to correct them.

Potential Research

With the advent of affordable sequencing, I would love to see a study that examined X-chromosome inheritance in a family of 6 or 8 generations.    How closely would it match the theoretical inheritance probabilities from the chart above?    It’s probably just a matter of time before this research is conducted.

P.S. - Feel free to use these charts, but please give [Dr. Bettinger] proper credit.

© The Genetic Genealogist


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