Sunday, December 11, 2022

DNA, Centimorgans, and Me

One of the joys of having been climbing my own family tree in these days of rapidly-advancing technology is learning more about my ancestors based on that technology and how they are used by successful ancestry-related companies.

As a reader might recall, I have been a paid member of one of the most successful genealogy companies,, for many years. Because my ancestors only relatively recently arrived in the United States--my grandparents arrived in the 1920s from Scotland (via Quebec), Ireland, and England--most of my ancestral documents are found in those countries, necessitating my paying a higher price for worldwide access. It has been worth it for one reason: my family tree data gets updated regularly as more and more people join and submit their own family information and DNA samples for analysis.

Recently, my youngest daughter submitted her own DNA sample at my request. Her results came back, and as expected, she now shows on my family tree as a daughter with 3,488 centiMorgans (cM, for short) and with 50% shared DNA, as expected; the other half of her DNA comes from her biological mother. In my daughter's case, I was not expecting surprises, nor was I surprised by any result I found.

But her results and those written by one of my favourite writers about his own family seem to conflict a bit. This is most likely due to my own lack of knowledge, so it led me to a more in-depth study of what a centiMorgan is because I am not a scientist, and my knowledge of genetics comes from a time long ago. Perhaps a brief history of the process is relevant.

Deoxyribonucleic acid, DNA, is a polymer composed of two polynucleotide chains that coil around each other to form the now-famous double helix. The polymer carries genetic instructions for the development, functioning, growth, and reproduction of all known organisms and many viruses. [That is a lot of scientific jargon; using your favourite search engine will help you understand the terms if you want.] DNA has been studied by biologists since the mid-19th century. As the knowledge base expanded, so did interest in the basic helix of all life, leading to modern times and the two most famous scientists in the field, James Watson and Francis Crick. Watson, an American geneticist and molecular biologist, and Crick, a British biophysicist and molecular biologist, were affiliated with the University of Cambridge in England. Together with other scientists and graduate students in 1953, they were successful in obtaining the correct structure of DNA of the human genome. This one event has led to rapid advancement in understanding human cellular biology and genetics, the causes of disease and afflictions, the creation of drugs to combat them and overall increase the quality and length of human life. Simply, the massive international genome project was one of the most important endeavours in human history.

DNA is a very complex double helix; it has been estimated that if the entire collection of DNA strands in a human body were laid out flat in one line, the distance would be equal to that of several trips to and from the sun. But modern advances in the field have shown that DNA is not limited to Earth-bound physical bodies or items. In 2021, scientists at Queen Mary University in London showed for the first time that DNA can be collected from the air, a finding that could provide new techniques for forensics researchers and investigators, and anthropologists; it might even help scientists understand the transmission of airborne diseases like COVID-19.

Though not without controversy, the discovery of the complex secrets in the double helix has also helped accurately identify everything from thousands of years old human fragments to the victims of modern homicides. It has also helped match humans with their long-ago ancestors and more-recent relatives, which is where my interest is.

This is where the function of centiMorgans comes in. The National Genome Research Institute defines it as "a unit of measure for the frequency of genetic recombination. One centimorgan is equal to a 1% chance that two markers on a chromosome will become separated from one another due to a recombination event during meiosis [which occurs during the formation of egg and sperm cells.] On average, one centimorgan corresponds to roughly 1 million base pairs in the human genome." The name came from Thomas Hunt Morgan, an American geneticist who worked on fruit flies (didn't we all in high school biology?), a commonly-used model organism for genetic research.

Quite simply, a centimorgan is a measurement or rate of how often something occurs. In genealogy, a higher cM means a greater likelihood of being more or less closely related. For example, in the example above, my biological daughter has a cM of 3,488, while a 4th-6th cousin from one side of my family (none of which I have accurately identified yet) would have as few as 24 cMs. 

In the case of genetics and genealogy, size matters.