DNA: We All Have It

Where did we come from?

A schematic of a DNA molecule. (Public Domain image from Wikimedia commons.)

I just got the results from my Genographic DNA study, and I thought I’d share them. To start with, I thought I’d explain a little about DNA, and how the Genographics study uses it.

DNA is the abbreviation for deoxyribonucleic acid, and thank goodness it has a widely used abbreviation! If you enlarged a molecule of it enormously it would look rather like a twisted ladder with four different kinds of “rungs.” The genetic information is coded by the order of these kinds of rungs. I’m not going to get technical here, but if you want more information have a look at Wikipedia.

DNA is the information-carrying part of chromosomes. Most normal human beings have 46 chromosomes: 23 from the mother and 23 from the father. (The rare exceptions generally have some kind of medical problem, such as Down’s Syndrome which results from having three copies of one particular chromosome.) These chromosomes are in the nucleus of just about every cell in your body. In addition, the cell body has structures called mitochondria which are essential for metabolizing nutrients. These mitochondria have their own DNA, but since the cell body and mitochondria come from the egg cell, the mitochondrial DNA is inherited only from the mother.

There is one pair of the 23 which is special, called the sex chromosomes. These chromosomes come in two forms, X and Y. The Y-chromosome determines maleness, and seems to have little other genetic information. The X chromosome has a normal complement of genes. A normal woman has two X chromosomes. A normal man has one X (from his mother) and one Y (from his father.) Individuals with other combinations such as XXY occur rarely. The important thing for us is that Y chromosomes are passed only from male to male.

Thus there are three types of genetic analysis which can be done: nuclear, mitochondrial and Y-chromosome.

The nuclear DNA is what makes me, me and you, you. It comes from both parents, and is remixed in every generation. Your nuclear DNA can be compared with that of indigenous populations throughout the world, and interpreted as what percent of your ancestry came from what area.

Mitochondrial DNA is passed almost without change from mother to child. Thus my mitochondrial DNA is the same as my mother’s mother’s mother’s mother’s mother, a woman named Ellen Carr. Changes do occur, very slowly, and these can be used to track back how my maternal line traveled out of Africa.

I don’t have a Y-chromosome, so the Genographics results don’t give me any information on my father’s line. A cousin, a son of my father’s brother, has been tested by another company, so I know I am in Bowling Group 6. But I don’t have the pathway my paternal ancestors took out of Africa.

Next week I’ll retell the story Genographics told of my maternal line.