DNA Genealogy – Seven Terms You Need To Know
Did you get your DNA results backs and that has lead to DNA genealogy terms that you have never heard of before? Here are definitions that can help you understand most of what you read when trying to get answers from your DNA results.
a centimorgan (“cM” for short) is a measurement of genetic distance or linkage. It is not a physical measurement of length, but a statistical one: it measures how likely it is that a particular combination of chromosomes will recombine as they pass from parent to child. The higher the number, the closer the relationship. One centimorgan equals a one percent chance that a marker on a chromosome will become separated from a second marker on the same chromosome due to crossing over in a single generation.
You will see this one a lot. SNP, single nucleotide polymorphism or simple nucleotide polymorphism, is abbreviated to just SNP (pronounced snip; plural snips), are the most common type of genetic variation among people. Each SNP represents a difference in a single DNA building block, called a nucleotide. For example, a SNP may replace the nucleotide cytosine (C) with the nucleotide thymine (T) in a certain stretch of DNA. There are about 10 million SNPs in the human genome. The 3 largest DNA testing sites test for about 700,000 SNP’s. I may have a string of ATTGGAACA and you may have the same string in the same location and if the matching string is long enough, then it might indicate that we are related.
Each gene in your DNA is made up of two variants called alleles, which code for particular traits like hair and eye color. And because each parent contributed one allele of each gene to you, studying patterns of inheritance through alleles can help you guess which allele came from which parent and maybe even what alleles you might pass down to your own children.
Mirroring is using the family tree of your closest DNA relative and attaching yourself to their information and/or using their family tree as yours. You set up your own private tree on a site like Ancestry, then look at the family trees of your closest relatives. Then you can copy their relatives and put them in your tree as your relatives and see what hints show up. You can also attach your DNA results to someone else’s tree and see what hints show up. I was lucky enough to have a 2nd cousin show up on Ancestry and allow me access to her tree and she filled me in on names of her parents which I couldn’t see because Ancestry blocks information about living people. This was a huge break though for me.
Phasing is the process of determining the parental source of each allele. Phasing is done either by comparing a test-taker’s results to the results of his or her two parents, or by comparing a test-taker’s results to the results of one parent and several other family members. Phasing offers several benefits, including the ability to determine exactly what side of your family tree a genetic cousin matches you.
This is a technique used to compare autosomal DNA in segments of three or more people to try to determine where those people got that particular segment of DNA. Using tools like DNA Match.com you can run triangulation reports. You may be related to Jane and to John. Jane may be related to you, but not to John. Triangulation will show you the people in common you are related to.
Genetic Family Tree
Your genealogical family tree (the one we’re all familiar with; names on a chart) is much different from your genetic family tree, which is all about the DNA you inherited. Each person inherits some DNA from their father and some from their mother – but DNA is not all equally shared. Because you may receive more from your mother than your father – or vice versa – and they might have gotten more from one parent than another, it’s possible to have DNA that disappears from your line within a few generations. It also means that your own sibling might have different DNA results than you, as their combination might be completely different. This is why my sister and I each look like our grandmother – only I look like my dad’s mom and my sister looks like our mom’s mother.