Autosomal DNA (atDNA) is a great tool for identifying and confirming shared ancestry because of the way it is passed down to each person. We can use atDNA to explore and find additional relatives that we may not have previously known about by taking a DNA test through providers such as Ancestry or FTDNA and allowing our atDNA to be matched with others. This could assist us in breaking through a brick wall by discovering lines we were not previously aware of. We can also use atDNA to help us confirm our suspected ancestral lines by having others within that line test their atDNA. Adoptees can use atDNA to find family members which could ultimately lead to finding their biological parents.[i]
We can do this because atDNA is inherited from both parents, who in turn inherited their atDNA from both of their parents, and so on down the line. A child receives approximately 50% of their DNA from their father and the other 50% from their mother, who in turn each received approximately 50% from their parents.[ii] This process is, however, not linear. The inheritance occurs within our 22 (44 matches pairs) of chromosomes using a process called recombination (you have 46 total chromosomes, the last 2 are the XX or XY that determine the sex). This process is an independent one that occurs during meiosis, where each of the sex cells, one from the sperm and one from the egg, combines and recombines to receive a random assortment of genes from the parent[iii].
Figure 1 Recombination
So, that means that while, in theory, you could receive 25% of your atDNA from each of your grandparents, you most likely do not because of the random nature of the recombination, as shown in the image below.
Figure 2 Randomness of inheritance pattern
Autosomal DNA matches may confirm a relationship, but not the type of relationship. Due to the random way the recombination works, it is possible that you will not share DNA with a relative, particularly past 5-7 generations. Matching tools used for atDNA can show a match, but not whether that match is on the paternal or material side, therefore further analysis is always necessary when a match is found. A further limitation is that it is also possible that you may have shared atDNA due to sections being IBS (identical by state) or identical by chance rather than IBD (identical by descent).
[i] University of Strathclyde Centre for Lifelong Learning (2021) Understanding autosomal DNA testing for genealogy: a beginner’s guide. Class 1 Introduction to DNA Testing.
[ii] Bettinger, Blaine T. (2019) The Family Tree Guide to DNA testing and genetic genealogy, 2nd Edition. Penguin Publishing Group.
[iii] University of Strathclyde Centre for Lifelong Learning (2021) Understanding autosomal DNA testing for genealogy: a beginner’s guide. Class 2 Autosomal DNA testing.