To understand genealogical data, we must first learn about the role of DNA. It is the primary molecule of life and is responsible for the development and growth of every living thing.
The best way to describe the amount of genetic information stored in DNA is to picture a library full of books. If you took every line of copy in every page of every book and you cut it into strips and taped them one after another, end to end, you would have the equivalent information that DNA stores in our bodies.
With DNA all this information is organized into shelving called chromosomes. In the library you seek information by alphabet then shelf number then page and finally paragraph. The Human Genome Project did essentially the same thing in 2002, by creating a “card catalog” for all the genetic information in the body.
The biggest difference between the way books are cataloged in a library and genetic information is stored in DNA, is there are two copies of the information in each cell; one copy comes from the mother and the other comes from the father. Both copies are nearly identical which is how paternity is determined.
The biggest difference between males and females lies in the chromosomes. The 23rd pair consists of an X and a Y. Females have two “X”s and males have an “X” and a “Y”. When a baby is conceived the mother always passes down an X, but the father can pass down either an X or a Y; X for a baby girl and Y for a baby boy.
To trace ancient ancestral roots, genealogists look at the DNA with low rates of change or mutation, making genealogical data a reliable but imperfect source of information from up to 150,000 years ago.