Why are cousin and great-grandfather the same kinship?
The popularity of commercial genetic research is also growing in Poland, particularly among people involved in genealogy. What makes people want to check what they have “written” in their genes? Does this curiosity grow in proportion to the mysteries – an unknown ancestor, illegitimate children, or simply unknown origins? Descendants of emigrants probably ask themselves many such and similar questions. Portals where you can publish your family tree (Ancestry, My Heritage) are becoming very popular.
There are also portals where, in addition to your family tree, you can order a DNA test so that your results can be compared with other users (23andMe, FTDNA, Living DNA, Genesis GedMatch). Probably most accounts on these portals belong to people whose ancestors emigrated across the Atlantic Ocean at the turn of the 19th and 20th centuries.
Genealogy and genetics
Will the result of such a test give an answer – how many percent of me are Polish, Hungarian or Jewish? Will genetic testing replace traditional genealogy? Do they complement each other? Genealogy is both the search for ancestors and distant relatives. Probably everyone who has ever tried to draw their family tree has asked themselves a question – and who was before? Properly aroused curiosity opens the door to an endless adventure. Traditional genealogy seems to have boundaries where written sources end. Even if some documents from the 16th century survived, we can stand eye to eye with 15 different Bartek in one village. Well, how do we know who’s who? Names were just being created, and if someone was already known as Bartek from Cracow, how could his son or father be called? Jan from Kleparz, Marcin from Kraków, Wojtek from Młyn? But what are the limits of genetic research? Each person has 23 pairs of chromosomes, with thousands of data about ourselves stored on each one. The principle of inheritance is simple. The parents each pass about half of the genetic material to their child. Just the natural process that controls evolution. From time to time there will be some sort of mutation, meaning a change in the DNA sequence. This makes each of us different, but also a bit similar. The autosomal test is just looking for such similarities in the DNA sequence. This is where the first frontier of genetics comes in. If our “cousin” hasn’t taken the test, then we have no one to compare ourselves to. In the Christian tradition, all humans descend from the biblical Adam, so in a way, each of us got something from Adam. How cliché that sounds. If we claim that we all have a common ancestor then what is the purpose of genealogy? And here we probably come to the heart of the matter – we ask not only about ancestry, but also the degree of kinship. We want to be able to count it.
1 – me, 2 – parents, 3 – grandparents, 4- great-grandparents, 5- great-great-grandparents, etc. The appetite grows as you eat. Wouldn’t it be phenomenal to be able to count to 11, for example? The 11 generations back in time can be traced to ancestors who lived in the 18th or even 17th century.
Genetics uses an alphabet consisting of 4 “letters” – A T G C. These are the first letters of the names of the nitrogenous bases that form the DNA sequence e. g. AAAGCTTA. A – adenine, T – thymine, G – guanine, C- cytosine. DNA sequences store information about the structure of the proteins that build our bodies, which are passed on hereditarily. And that is the key information – it is transmitted hereditarily. It is difficult to read an ancestor’s name from these symbols, although it probably won’t be long before their appearance can be determined. Just which ancestor exactly? Father, mother, paternal grandfather, paternal grandmother, maternal grandfather, maternal grandmother? We have 8 great-grandparents, 16 great-great-grandparents. . 32, right. . 64, 128, 256, 512, 1024. We can count over a thousand ancestors as far back as 10 generations. The question is which ancestor, 1 out of 1000, gave us exactly the one fragment that is preserved in our chromosome. In my case, many thanks to that ancestor who passed down the mole on the finger of my right hand. As a result, I had no problem in elementary school distinguishing between my right and left hand.
So how can we get even slightly closer to answering “Who’s who in the DNA chain”? The easiest way would be to test your grandmother or grandfather and see if we have the same information stored in a given piece of DNA. But how do you research an ancestor who lived 200 years ago? Theoretically, if we know where his human remains are buried.
And this is where another problem arises. I don’t know where my great-grandmother, who died in 1942, is buried. So if we can’t test it directly then how? We need our cousins (genetic of course). The Autosomal Test involves a comparison of “ATGC” sequences. If there is the same order of appearance of the same “letters” then there is a high probability that the algorithm will give a positive result – there is a similarity. The raw data looks roughly like this:
In the picture above, we see a fragment of the results that were deciphered through the DNA analysis process. At first glance you can see that the two passages are different. However, if we consider the fact that at least one match at a given location in two individuals is sufficient to speak of similarity, we find that these results may belong to relatives who inherited the same fragment from a common ancestor. Similarities are highlighted in green:
This is how the autosomal test comparison algorithm works in a nutshell. Now consider that the smallest human chromosome has about 47 million such pairs, and the largest about 250 million. . . and there are 23 chromosomes. You need a really good computer to compare all this in a reasonable time.
Common DNA segments and centimorgans
Now a few words to make it even more complicated. The chromosome consists not of individual segments of ATGC letters, but of pairs of them (as in the earlier figure). This means that each pair has double information: one “letter” comes from the father and the other from the mother. The problem is that we currently can’t tell which is which. Therefore, the algorithm indicates similarity if at least one of the “letters” matches. So if we return to our example, we can sort the results so that in the top “line” are those “letters” that repeat in both people. If they are indeed cousins, then this fragment is inherited from a common ancestor.
Genetically, we are as related to our parent as we are to our child, to our uncle as we are to our nephew. Why? Because we have, on average, only the same common DNA fragments.
So it is worth learning a new way of counting kinship, right alongside those under civil law and church law. Instead of IV degree of relatedness we can say 123. 4 centimorgans. Centimorgan (cM) is a unit of measurement that is used to determine the distance between areas occupied by a gene. The longer the segment and the more areas are similar (i.e. at least one letter matches), the more common centimorgans we have. With parents it is about 3500 cM (50%), with grandparents about 1750 cM (25%) etc. For relatives in the lateral line – siblings, cousins – the amount of cM can vary considerably. Since they had the same ancestors, they may have inherited the same as well as other fragments of DNA. That’s what all the fun is about, to find those common ancestors in common fragments. If we don’t know the degree of relatedness to our genetic cousin, it is the comparison of our ancestors that can provide the answer if we find those in common. This common ancestor could even be someone who lived as far back as the 18th century. It is also worth remembering that everyone inherits a different set of genes, so a brother and sister may inherit different genes from ancestors who lived in the 18th century. So, the more we are able to identify closer relatives, the more chance we have of discovering much further cousins whose roots go back much further than metric books.
The Blue Grandpa
There is nothing better than a picture or a diagram, so if the description so far is not enough for you to understand the basics and start playing with genetic genealogy yourself, I will put it even more vividly. I hope that specialists in this field will not accuse me of oversimplification. However, as a physicist by training, I like to present the most difficult complexities of the laws of science in the most illustrative way possible. Let’s compare two siblings and their shared “blue” grandfather. Let’s skip the whole question of analysis and get straight to the results. To some extent the algorithms do this automatically, so the whole job of DNA analysis is much simpler. However, it is worth knowing what it looks like from the inside. We used the DNA Painter tool and determined the fragments that were inherited from all the grandparents. For simplicity, each piece has been colored in a different color.
Nice, colorful and diagrammatic, but what’s the point? Let’s go through it step by step.
So start with the “blue” grandfather. Crossover may occur in the recombination process (exchange of genetic material). The simplest way to put it is that it is an exchange of stretches of DNA chain. In the figure below, a red line marks the site of gene disjunction. Some of the information disappears irretrievably, half to be exact. There will be a “blue” section on one side, and a “light blue” section on the other (in other words, the “letters” from the top “line” on one side, and the “letters” from the bottom “line” on the other side).
The resulting “half-chromosome” is passed on to the son, and he receives the other half from the “green grandmother”.