Genetics and DNA Basics Part 2

The X and  Y factor

One of the most useful DNA tests available is only available for males and it tests the Y chromosome. The  Y chromosome is passed down from father to son virtually unchanged. Combining that stable exchange of DNA with the western tradition of using male dominated surnames,  Y DNA is very useful for breaking through brick walls,  determining family groups and for determining migration patterns.

There are two main types of Y DNA tests –  STR,  which stands for single tandem repeat,  and NGS where they look at the entire Y chromosome looking for minor mutations called SNPs. Both are very useful and while they overlap in areas of coverage,  generally STR tests are really useful for research in the past  200-1000 years. NGS tests will tell you the entire history of your lineage path from Africa to today.  SNPs also can be identified that are unique to a specific family group and can be used as an inexpensive screening tool to determine if two family groups are related.

As I said,  STR stands for single tandem repeats and they are known locations on the  Y chromosome where those nucleotides repeat.  An example would be a spot on the  Y chromosome where  CAAC is repeated numerous times in a row. CAACCAACCAACCAACCAACCAACCAACCAACCAAC would have a STR value  of  9.  Those known locations can be compared and family relations can be determined by that comparison. The standard Y STR tests are done with  37, 67 and  111 markers. Obviously the more markers tested,  the better the accuracy. Custom STR tests are also some of what law enforcement uses to determine genetic evidence in cases.

In our family,  here’s an example. We belong to what’s known as the  Adams 004 family group and we have roughly 20 males that have been tested with sufficient markers to know that we all share a common male ancestor Adams. We’re still on the quest to determine whom he is. We all closely match each other on at least 37 of those markers. How closely we match and on what markers can help narrow down the time-frame for us to look for that common ancestor.

The second type of  Y DNA test looks at the entire Y chromosome and maps it out completely. It compares what it finds against a base genome and points out minor mutations along the chromosome. Those mutations,  or SNPs,  are used to determine branches in the haplotree for humans. Using those tests in combination with standard archeological methods, scientists are gaining a greater understanding of the human migration patterns out of Africa. The text is also very useful in determining male haplogroups.  We have determined that we are part of  R1b-U106 and below that R-L1. We’ve even determined a number of SNPs that are unique to our family group and are very useful to use as a screening tool to determine if other male Adams are cousins of ours or not. If you’re a male Adams and have a family history that originated in Scotland,  Ireland, North Carolina or  South Carolina – give me a shout to see if we are related.

Mystery X

The X chromosome presents more mysteries to me than solutions. I’m sure genealogists with far more experience than I have can figure out how to use it in genealogies but it befuddles me. I’ll cover what I know.

Men receive a Y chromosome from their fathers and an un-recombined X chromosome from their mother. The reason it’s un-recombined is because there is no paternal X chromosome to recombine with. So my entire 197.1 cM of X chromosome was passed down to me from my mother.

Women get X DNA  from both their parents. The odd part is that sometimes those chromosomes recombine just like regular 1-22 chromosomes do. Sometimes, they don’t. There doesn’t appear to be any hard and fast rule.

The graphic below will illustrate my point.

This is a list of my closest X DNA matches ona website called gedmatch. (I’ll cover that in another post) I have two different DNA profiles on gedmatch so the first match is to myself – a full 196.1 cM. My youngest sister is my next closest match. She and I share 78.2 cM of X DNA. That means she got 314.0 cM of X DNA from my father and only 78.2 from my mom.

My middle sister Pam was even stranger. She had three children – all males. So her X DNA should have been passed down to her children un-recombined. In theory, all three of her children should share the exact same amount of X DNA with me. Nope. Two out of the three do – at 23.5 cM. Her youngest son, however, matches me at 37.9 cM. Very weird. The other thing you’ll notice is that there are 11 other matches up there that share roughly as much X DNA as my nephews do. None of these people are close cousins – the closest is estimated to be 4 generations until we hit a common ancestor. As I said, X is weird.

The bottom line for me is that I use X DNA for one thing and one thing only – it lets me know for sure that the match is on my mother’s line. If you’d like to know more about X DNA and its weirdness, Roberta Estes has an excellent genealogy blog. Here are two of her articles about X DNA.

That Unruly X….Chromosome That Is

Using X and Mitochondrial DNA Charts by Charting Companion

What about mitochondrial DNA?

“Although most DNA is packaged in chromosomes within the nucleus, mitochondria also have a small amount of their own DNA. This genetic material is known as mitochondrial DNA or mtDNA.

Mitochondria (illustration) are structures within cells that convert the energy from food into a form that cells can use. Each cell contains hundreds to thousands of mitochondria, which are located in the fluid that surrounds the nucleus (the cytoplasm).” from  – mtDNA Basics from NIH

Mitochondrial DNA is only passed on to children by their mother. The difference is that women pass mtDNA to both their sons and daughters. Since only women pass on their mitochondrial DNA, it allows us to trace the female lineage much like we do with males and the Y DNA. The problem is that each generation, the surnames change. Add to that fact, mtDNA is less specific at the haplogroup level (requires coding regions to narrow down) and you have a fairly blunt tool to use for genealogy. I’ve looked at mine and the earliest known female ancestor I have is potentially Martha Perrin born in Henrico, VA in 1694. In my list of matches, there isn’t a single surname that is in common with me. The closest match I have on FTDNA has as his earliest known female ancestor a lady that was born in Virginia in 1810. The next 7 or 8 next closest matches I have list no female ancestors. Only slightly more than 50% of my mtDNA matches even list a female ancestor and most don’t go back past about 1800. For me, I use the female haplogroup for overall migration patterns and that’s about it. Earlier in this post, I linked to Roberta Estes blog. She has some hints on using mitochondrial DNA and how to chart it all out.

My next post, I’ll cover the major testing companies and options out there.