Genetics and genealogy: principles of genetic inheritance

DNA testing for genealogy depends on the process of inheritance. Every human being inherits different parts of their DNA in varying ways from their parents. Understanding the principles of how this inheritance takes place is helpful if you’re wondering which type of DNA test you should take to answer specific research questions. This short video aims to help you understand these important principles. Human DNA is contained in 23 pairs of chromosomes. 22 of these pairs are called autosomal chromosomes. And the 23rd pair are the sex chromosomes. This video shows how the inheritance of the autosomal chromosomes is different from the inheritance of the sex chromosomes.

You can see here that around 50% of your autosomal DNA is inherited from your father shown in blue. And 50% from your mother shown in red. The sex chromosomes work differently. These are named X and Y with men carrying one X and one Y chromosome and women carrying two chromosomes. Men inherit the X from their mother and the Y from their father, as you can see in the case of the son on the left hand side of the screen. Women inherit the X from their mother along with a second X chromosome from their father, and this is shown for the daughter on the right hand side of the screen.

Almost all of the Y chromosome is passed from father to son, but the X chromosome which is passed on by the mother is a recombination of the X DNA, which she has inherited from her two parents. This is shown in the next video, which focuses on the inheritance of the X chromosome from the mother. You can see that the X chromosomes from the mother’s parents recombine, and this mixture of X chromosome segments is passed on to the daughter.

The significant points to draw from these principles are that autosomal DNA is inherited from all of our recent ancestors, X DNA is inherited from some, but not all of our ancestors, and Y DNA is inherited only by an unbroken line of male ancestors. For the moment, we will focus on autosomal inheritance. The amount of autosomal DNA inherited from a specific ancestor reduces by around half at each successive generation. In the video, you can see that children inherit around 50% of that autosomal DNA from each parent. In later generations, 25% is inherited from each grandparent, and 12.5% from each great-grandparent. You also need to know that the segments of DNA inherited by siblings are different.

So the 50% inherited by one sibling from their mother will be different from the 50% inherited by another sibling. This can be displayed in a utility called a chromosome browser. Using an example for a male, first you can see the chromosomes showing the segments of autosomal DNA inherited from each parent, yellow for the mother’s DNA and blue for the father’s DNA. The next image shows the segments for the same male with those shared with the father coloured yellow, those shared with the mother coloured blue, and those shared with a sibling coloured green.

Now that you have some understanding of the scientific principles behind autosomal inheritance, you’re ready to look at how autosomal test results are interpreted and how they can be used to answer some genealogical questions.