Sunday, April 20, 2014

Which Ancestors Are Talking?

We all understand that our DNA speaks to us to inform us about the past, the present and the future. As genealogists we are primarily interested in discovering what it has to tell us about our past. To be able to integrate this information with other sources of information, we need to be able to answer two basic questions. What are our ancestors saying and which ancestors are talking?

We will be expanding our knowledge of what our ancestors are saying for the rest of our lives as more and more of the human genome is understood. We can now have a more complete knowledge of which of our ancestors are talking even if we cannot yet fully understand what they are saying. A basic understanding of who is talking to us is one of the first essential building blocks toward becoming accomplished genetic genealogists. 

When we have a genealogical question, we need to be able to understand which parts of our DNA might have information to answer it. Which parts of our DNA give voice to which parts of our ancestors? It comes down to understanding the four unique inheritance patterns of four distinct areas of our DNA. 

By now most of you know that women carry three of these: mitochondrial (mtDNA), autosomal (atDNA) and X chromosome (xDNA). Men carry a fourth kind which I sometimes call "guY DNA" because it is what makes them a guy. If you understand how each of these components of your genome were passed down to you, you have the beginning of an understanding of which ancestors might have something to tell you about your family history. Whole books can and have been written on this subject so what follows below is only a summary of the Reader's Digest condensed version. 


Our mitochondria are not even in the nucleus of our cells. They float around in the fluid that surrounds the nucleus. Some cells contain thousands of copies of our mtDNA information. It has ONLY 16,569 locations. A complete mtDNA test can read the code at each of those positions. 

We all inherit our mtDNA from our mothers and only from our mothers. Men do not pass it on to their children. It is one of the two kinds of "celibate DNA." Celibate DNA is not mixed between with the DNA of the other parent but is passed down essentially intact generation after generation. Therefore it is good for tracing deep ancestry but only along one single line of our pedigree chart. In this case mtDNA would allow us to read information about our mother's > mother's > mother's line back to mtEve. Since it is the smallest "book" in our DNA library it is not a good tool for distinguishing subtle differences between siblings and between descendants of siblings who share a common female ancestor. This is your matriarchy talking without any direct influence from your male ancestors. 


The rest of our DNA is contained in 23 pairs of chromosomes. 

22 pairs of these chromosomes, the autosomes, share a common inheritance pattern. The two sex chromosomes each share unique patterns. 


As you you can see above, the y-chromosome is one of the smallest of the chromosomes. However, with about 58 million base pairs it is humongous compared with our mitochondria. We are still learning how to read the codes stored at most of these locations. yDNA is the other kind of celibate DNA. It generally is passed intact from father to sons generation after generation. It can retain its information for eons because it is not diluted by the DNA contributions of the mothers. However, since male siblings each inherit essentially the same yDNA from their fathers, it generally is not too useful in differentiating between the descendants of brothers. When you listen to your yDNA, it is your patriarchs talking without direct input from your maternal ancestors. 


By far the majority of your DNA is contained in your 22 pairs of autosomes. These are one of your two types of promiscuous DNA. In this case their promiscuity is both their endearing quality and the source of frustration as we try to read the information they carry. atDNA speaks to us to help sort out relationships that occurred in the last few generations. Occasionally we get a shout out from an ancestor much further back. However, the apparently random way that our parents pass down the DNA they inherited from our grandparents simultaneously tantalizes and confuses. When you listen to your atDNA, you are most often hearing your ancestors of the last four or five generations ALONG ANY of the lines of your pedigree chart.


The other type of promiscuous DNA is xDNA. You inherit it in a manner similar to the way your receive atDNA with one very important twist. Males do not pass along xDNA to male offspring. They pass along yDNA instead. Did I say males do not pass along xDNA to male offspring? Yes I am being redundant. Repetition is the key to learning. Many advanced genetic genealogists have trouble recalling the inheritance pattern for xDNA. If you remember the admonition in this paragraph, you will begin to be able to sort out relationships shared through xDNA. When you listen to your xDNA you will not hear any information from any ancestor who is earlier than a male to male blockage. For example you will not hear any information from your paternal grandfather's quadrant of the pedigree chart because he did not pass any xDNA to your father. This same principle must be applied again and again in other sections of your pedigree chart. 

Which ancestor(s) would you like to talk to today?

Take a look at your pedigree chart. Which ancestor(s) might be able to help you answer a relationship question if they shared the information in their DNA? How did you or a close relative inherit that ancestral DNA? Happy testing and analysis!

1 comment:

  1. Unfortunately, when it comes to autosomal DNA, endogamy seems to trump everything.