Identification of Genetic Relatives
Autosomal DNA
Autosomal DNA (atDNA) testing is a tool used by various testing companies to identify and analyze genetic relatives. atDNA is inherited from both parents. It is found on the first 22 chromosome pairs called autosomes. Before the autosome is passed down, it undergoes a recombination process where it is randomly shuffled; therefore, the child carries pieces of atDNA from their ancestors. Additionally, with multiple generations, the ancestors’ atDNA becomes more fragmented (Genetic Science Learning Center, 2016).
Identifying Mary and Susan as Genetic Relatives
A centiMorgan (cM) is a recombinant frequency unit used to measure genetic distances. Sharing a large number of cMs in common indicates the likelihood of sharing a common ancestor. Assuming that Mary and Susan share a segment of DNA on chromosome 5 that is 6.59 cMs long, they are not likely to be identified as genetic relatives in Family Tree DNA (FTDNA). cM values vary across different companies, for instance, depending on the genome build number. Also, FTDNA uses 100-SNP blocks. Mary and Susan’s possible relationship would be with fourth cousins once removed or third cousins three times removed (“Autosomal DNA statistics,” n.d.).
Mary’s and Susan’s Possible Relationship
Assuming that Mary and Susan share three segments of DNA on chromosomes 5, 12, and 14 that are 6.59cM, 51cM, and 19cM long, respectively, gives 76.59 of the total cM shared. Therefore, Mary and Susan’s possible relationship would now be that of third cousins; second cousins twice removed, according to the ISOGG Autosomal DNA Statistics table. On the other hand, looking at the Shared cM Project, Mary’s and Susan’s possible relationship would be 3C1R (third cousins once removed) (“The Genetic Genealogist,” 2015).
Conclusion
When identifying genetic relatives, it is important to carefully distinguish the distribution of shared DNA segments between the given relationships and the sharing of relationships for the given amount of DNA. Additionally, testing companies use cMs as measuring units to distinguish shared DNA segments for given relationships. However, cM values vary across these companies.
References
Autosomal DNA statistics. International Society of Genetic Genealogy Wiki. Retrieved 23 November 2021, from http://www.isogg.org/wiki/Autosomal_DNA_statistics.
Genetic Science Learning Center. (2016). Introduction to Molecular Genealogy [Video]. Retrieved 23 November 2021, from http://learn.genetics.utah.edu/content/basics/molgen/.
The Genetic Genealogist, Visualizing Data from the Shared cM Project. Thegeneticgenealogist.com. (2015). Retrieved 23 November 2021, from http://www.thegeneticgenealogist.com/wp-content/uploads/2015/05/Shared-cM-Project-Image-2.png.
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Question 
Now that you have read about how genetic relatives are identified by the testing companies and some tools that can be utilized to analyze these matches, let’s check your knowledge.
The following questions evaluate your understanding of the testing companies’ identification and analysis of genetic relatives.
Identification of Genetic Relatives
Read the following and, based on your understanding of autosomal DNA (atDNA), provide short written answers to the following questions:
Assume that Mary and Susan share a single segment of DNA. That segment of DNA is located on chromosome 5 and is 6.59 cMs long.