Wednesday, January 14, 2009

Our X chromosome, pt. 1

Blaine Bettinger, PhD. writes an excellent blog called The Genetic Genealogist which I would recommend to you for regular consumption.

On the chance that you might not, however, and because I'm a complete novice where the X chromosome is concerned, I'm going to copy two of his columns verbatim here.    What follows is entirely Dr. Bettinger's work as published by him on Dec. 21, 2008.

Unlocking the Genealogical Secrets of the X Chromosome

Most genetic genealogists have sent away their cheek swabs to learn about their mitochondrial DNA or their Y-DNA lines.    Others have explored their autosomal DNA for ancestral information, a field that is growing quickly and will undergo rapid changes as the price of sequencing continues to fall.

Now genetic genealogists are beginning to discover the ancestral information locked away in the X chromosome.    Indeed, X chromosome tests have been offered by companies such as Family Tree DNA for a number of years.    Armed with some of this information as well as the advent of SNP chip information from 23andMe and deCODEme, genetic genealogists are making new discoveries in this very young arena.

Inheritance of the X Chromosome

To help you understand some of the X chromosome data, I’ve prepared this short summary regarding the unique and interesting inheritance of the X chromosome.    Males, of course, have one Y chromosome from their father and one X chromosome from their mother.    Females have two X chromosomes, one from each parent.

The charts below trace back the inheritance of the X chromosome through the level of GGGGG-grandparents.    At that generation, a person has 128 ancestors.    Of these 128 ancestors, a male will have 21 people who potentially contributed to their single X chromosome (8 males and 13 females).    A female will have 34 potential contributors to her two X chromosomes (13 males and 21 females).    Note that I say “POTENTIAL” contributors because it is unlikely that all these ancestors are equally represented in the X chromosome   –   it is more likely that some ancestors are completely missing while others are well-represented.

What I found to be particularly interesting is that the number of X contributors at each generation follows the Fibonacci sequence of 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233… (HT: John Chandler).    A male will start with 1 X contributor and then follows through the sequence, while a female will start with 2 X contributors and follow through the sequence (although the numbers will be different if there is recent overlap in your family tree, as there is in mine).

Male Inheritance (click to enlarge)   –   Male contributors are in blue and female contributors are in pink:

Female Inheritance (Click to enlarge)   –   Male contributors are in blue and female contributors are in pink:


It is important to keep in mind that this investigation into the X chromosome is VERY new and thus can be confusing or unclear.    While I don’t recommend jumping into this area if you aren’t ready for the many changes, reversals, or dead-ends that will undoubtedly appear, I would encourage anyone who is interested in assisting these researchers contribute their own information if you feel completely comfortable doing so.

It will be very interesting to see how this field develops over the next few years.

P.S. – Feel free to use these charts, all I ask is that [Dr. Bettinger] be credited with a link to [his] blog.

UPDATE: Ann Turner has a text file of the Ahnentafel numbers of those ancestors who potentially contributed to the X chromosome, through 10 generations. If you are a male, be sure to start the ahnentafel chart with your mother.

© The Genetic Genealogist


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