The ability to use DNA for genealogical research is based on the fact that we inherit all our DNA from our ancestors in certain ways. There are three types of DNA and here we describe how each one of these are inherited:
Autosomal DNA makes up 99,5% of all our DNA. This DNA is inherited in a random mix 50/50 from both parents, who have inherited their autosomal DNA 50/50 from their parents etc. Therefore we all carry a mix of bits and pieces of DNA from our ancestors. But not from all ancestors, since the random mix in every generation makes DNA from many ancestors drop out.
Y-DNA is only carried by men. It resides in the Y chromosome that is inherited strictly from father to son. This means that every man is carrying the same Y-DNA as his straight line of fathers has carried for thousands of generations.
Mitocondrial DNA, or mtDNA as it is called, is only inherited from mother. It is carried both by women and men, but only women passes it on to their children. This means that every woman and man is carrying the same mtDNA as their straight line of mothers has carried for thousands of generations.
X-DNA is carried by both women and men in the form of one (men) or two (women) X chromosomes. A man inherits his X chromosome from his mother and a woman inherits one X chromosome from each parent. X-DNA is inherited according to a complicated pattern. The main principle is that it can’t be inherited through two men in a row. The consequence of this is that a woman and a man must have inherited their X-DNA from any of the coloured boxes in the chart below:
X-DNA charts created by Blaine Bettinger at The Genetic Genealogíst.
Y-DNA, mtDNA and autosomal charts created by Peter Sjölund.