The point at which I became a DNA geek.

The next part of the search for Philip Holmes involves a little bit of geekiness, and a slight detour.

At some point in the learning curve of using DNA matching in genealogical searches, I believe some of us cross the line from “enthusiast” to “geek”. For me I think it came when I discovered what is referred to as “the unique inheritance pattern” of the X chromosome.

The what?

Exactly. Not the kind of sentence I thought I would be writing when I first started delving into genealogy. Now it flows from the keyboard so naturally that I know I have moved into a different realm of understanding. I might even admit that I scare myself, ever so slightly, when I use phrases like that. I am not a scientist, a biologist or a geneticist, nor do I claim to be. In college I “majored” in English literature and language, which is probably why I can explain literary terms such as “trochee”, “synecdoche” or “enjambment” – (and which also likely confirms my place in geekdom).

In a nutshell – we have 23 pairs of chromosomes (and that is one reason why there is a testing company called “23andMe”). The first 22 pairs are referred to as “autosomes”. The 23rd pair also have a scientific name but are more easily explained by calling them “sex” chromosomes. The chromosomes come in pairs because we inherit one from each parent. The 23rd pair will determine if we are male or female.

X plus X = female.

X plus Y = male.

For various reasons, genealogists researching DNA matches don’t often use matches on the X. But in certain cases it can be significant and exciting because of the aforementioned “unique inheritance pattern.”

The what (again)?

In an even smaller nutshell, (within the first nutshell?)…this:

X DNA inheritance

Females get an X chromosome from each parent, but males only get X from their mothers. This means, if you have a large enough DNA match with a male on the X chromosome, and you also share some autosomal DNA with the match, you have to be connected through his maternal side. Because X can’t be passed down from one male to another, that automatically excludes certain lines of his ancestors from your search.  You can only be related to a male match on the X chromosome through one of his ancestors marked in green in this chart.

X DNA male inheritance.

(This chart and the one before it were taken from the genie1.co.au website written by Louise Coakley. )

We can sit and wait for days or months on end, fiddling with the trees of 4th cousin matches, working down from their great, great, great grandparents through the generations, with collateral lines increasing exponentially as we go. Then one day, we wake up, check our DNA matches and discover that the one match that we have long sought to break the mystery has finally appeared. In this case it was a fairly measly-looking potential 4th cousin match that turned up on Gedmatch. He shared both autosomal and X DNA with Carol’s mum. Importantly, he also did not seem to be a matching kit for what we called “the usual suspects” in this search – a large group of people we knew to be connected to Carol’s mum’s side of the family. And because this match was male, it meant that potentially we would only have to consider that small selection of ancestors in green in the chart. Of course we would still have to work out if he connected to any ancestor by the name of Holmes in London at the beginning of the 20th century. How hard could it be?!

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