DNA Basics

In humans, there are different types of DNA: Autosomal DNA, X-DNA, Y-DNA, and Mitochondrial-DNA. Autosomal DNA is the most commonly used type of DNA used for research, but the other's have value as well.

Autosomal DNA
In humans, 95% of DNA is autosomal DNA. It consists of 22 pairs of chromosomes and in each pair, one is from mom and one is from dad. The pairs of chromosomes are simply names 1, 2, 3, etc... 21, 22. They are mostly named by their size with chromosome 1 being the largest, chromosome 2 being the second largest etc. They are not perfectly named by their size, for example chromosome 22 is actually bigger than chromosome 21, chromosome 20 is slightly bigger than chromosome 19 etc. This is because as our understanding of DNA grows and improves, some chromosomes turned out to be a little bigger or smaller than we initially thought.

Autosomal DNA is the most important and most commonly tested type of DNA. A child will have exactly half of their autosomal DNA from each parent. However, a child will usually not have exactly 25% from each grandparent. A child could have 20% from one grandparent and 30% from another grandparent for example. This is because each child is randomly given half of their parent's DNA. When a new sperm or egg is formed, the pairs chromosomes in the parent line and exchange information, meaning segments are randomly cut out and switch places with each other to form new chromosomes that are a mixture of the previous ones. The child is then given one chromosome from each pair and the other is discarded. This happens in both parents so that the child receives one full set of chromosomes from each parent.

Sex Chromosomes (X and Y)
Sex Chromosomes: There are 22 pairs of autosomal DNA. The 23rd and final pair are the sex chromosomes which come in two variants, X, and Y. Females have two X chromosomes, and males have an X and a Y. The X chromosome is about as big as chromosome 7 and the Y is about as big as chromosome 21. In other words, the X chromosome is about 3 times bigger than the Y chromosome. Because they are so different in size, during recombination in males, the two try to line up at but can only perform recombination at the tip (in females the two X chromosomes recombine without any problems). The father gives the child all of his X or all of his Y. If he gives X, the child becomes a biological female if he gives Y the child becomes a biological male. Because of this rule, both have unique inheritance patterns.

X-DNA: X DNA is similar to autosomal DNA except that a man never inherits it from his father. This allows some lines to be eliminated based on the presence of matching X DNA. For example, if a man shares X DNA with another person, that person must be related on the man's mother's side. A female inherits X DNA from her father, but all of that DNA comes from her father's mother (her paternal grandmother). A female will never inherit any X DNA from her paternal grandfather. All of the major DNA companies test the X chromosome as part of their package, but only 23andme reports matches on the X chromosome.

Y-DNA: The Y chromosome is only passed down father to son and only males have a Y chromosome. The Y chromosome does not go through the recombination process so the only way it can change is through spontaneous mutations. These mutations are usually harmless and the only way you can know if you have one is by taking a Y-DNA test. Ancestry, 23andme, MyHeritage, and Living DNA all test the Y-chromosome as part of their autosomal package and 23andme even predicts your haplogroup, but only FamilyTreeDNA sells specialized tests that generate lists of genetic relatives using the Y chromosome.

Y-DNA testing has several advantages for genetic genealogists. It mutates at a faster rate than mtDNA and at a rate that is more useful for genealogists. By comparing the Y-DNA of two individuals it is possible to determine how closely the two are related on the direct male line. Anyone who matches your Y-DNA test is related on both your and their direct paternal line. The common ancestor will always be a man who had at least two sons, one of which you descend from and the other that the match descends from. Because Y-DNA can only change through mutations, it can be used to find relatives on the direct male line up to 25 generations back whereas autosomal DNA is usually only helpful up to about 5 generations back. Because Y-DNA and surnames are usually both passed down father to son, Y-DNA surname projects exist that try discover how everyone with the same surname is related.

The biggest disadvantage to Y-DNA research is far less people have taken Y-DNA tests than they have autosomal DNA tests. The only way to overcome this obstacle is for more people to test. The other disadvantage is that Y-DNA can only reveal information about your direct male line. If you wanted to use Y-DNA to research your mother's father's surname, for example, your Y-DNA would not work. You would need to test your maternal grandfather or another close relative related on the direct male line.

Mitochondrial DNA


Mitochondrial DNA (mtDNA): Mitochondrial DNA is unique. The rest of human DNA is located in the nucleus of the cell and is divided into chromosomes. Mitochondrial DNA on the other hand resides in the cell's mitochondria (the part that provides most of the cell's energy) and it is connected in a circle just like bacterial DNA. It is also the smallest portion of human DNA being about only 16,000 base pairs long. In other words, it is about one third the size of the smallest chromosome. These unique properties cause mitochondrial DNA to be have a special place among genetic genealogists. One advantage of mtDNA are that because it resides outside the nucleus, it can only be inherited from the mother. Sperm only contains a cell's nucleus and anything outside of that only comes from the mother's egg. This means that it does not go through recombination and testing your mtDNA can reveal information about your ancestor's on your direct maternal line. A match on an mtDNA test must be related through your mother's direct maternal line and their mother's direct maternal line until eventually you will find a female common ancestor who had at least two daughters, one of which is your ancestress and the other is theirs. The other major advantage to mtDNA is that it decomposes slower than the rest of your DNA so it is more likely to be used for solving cold cases when the person of interest is no longer alive to test. It was by using mtDNA that the remains of Richard III were conclusively identified, for example.

One major disadvantage to mtDNA is that it mutates at a slow rate. A person whose mtDNA perfectly matches yours could be related through a common ancestor that lived anytime within the past 500 years. Also the direct maternal line usually has a different surname at every generation so surname projects are useless.