DNA Tutorial

Each cell in your body contains DNA (DeoxyriboNucleic Acid) which is the genetic blueprint which makes you who you are. Most of your DNA is contained the nucleus of the cell and is therefore called nuclear DNA. Additional DNA is contained in each of the small structures of each cell called mitochondria.

A DNA molecule is composed of two strands of sugar and phosphate molecules that wrap around each other to resemble a twisted ladder. This unique arrangement is referred to as a "double helix". The two strands of the helix are held together by nitrogen containing chemicals called "bases" in a similar way that steps of a ladder hold the two sides together. Each rung of the ladder is made of either adenine (A), thymine (T), cytosine (C), or guanine (G). The order of these bases specifies the genetic instructions which determine everything from eye color to gender. From the Human Genome Project we learn that the DNA found in the nucleus of each cell contains over 3 billion bases.

Nuclear DNA is arranged into paired thread like segments called chromosomes. In 22 of those pairs, members of each pair are identical and are derived from each of parent. Because these chromosomes are composed of random segments from each parent, it is difficult to use this DNA to research previous generations. The 23rd pair, however, is different. In females, the 23rd pair consists of two "X" chromosomes. In males, the 23rd pair contains an "X" and a "Y" chromosome. It is the "Y" chromosome that produces maleness. The important thing for genealogists is that the "Y" chromosome is inherited only from the father. This means that a man's "Y" chromosome will be like is father's which will be like his father's which will be like his father's.

The only difference between "Y" chromosomes in each generation may be an occasional mutation that will be found on one or a few locations called markers. These mutations occur randomly but at known average mutations rates. The pattern of mutations reveals how distantly related two men may be.

Y-chromosome testing will not tell you who your relatives are but it may tell you how many generations ago you will find a common ancestor with someone else who has tested his DNA.

Mitochondrial DNA (mtDNA) can be used in a similar way to trace one's maternal ancestors. mtDNA is inherited entirely from one's mother. Both men and women can trace their maternal lines using mtDNA; however, it is not as useful as the Y-chromosome since its slower mutation rate can only distinguish different ancestral lines over a very long time period (up to thousands of years). The Alsup/Alsop DNA project only uses Y-chromosome DNA analysis.

For more information on the Y-chromosome see the excellent tutorials on the Blair DNA Project web site and the FamilyTreeDNA web site. Additional information can be found on our Links page and our References page.