DNA Advisor Karen Evans loves helping fellow family historians piece together the puzzle of the past using DNA. Here she shares answers to some of those key questions that crop up time and again. Are you ready to get crunching the DNA numbers?
First of all I want to make it clear that I am not a mathematician. I just about scraped O level Maths and look aghast at my son currently taking A levels in Further Maths and Physics – if it wasn’t for DNA I would think I took the wrong child home from hospital!
However, at some point, understanding the numbers in DNA will actually make your results easier to use and help you get the most from them.
So here are some of the questions I’m regularly asked.
You will have plenty of time, won’t need a calculator and no one will be checking your answers.
(Yes, I did used to be a primary school teacher many moons ago; but no, you are not at school. These mathematical puzzles are strictly for family history
purposes only).
QUESTION 1
If two people take a test and are related, how likely are they to show up on each other’s match list?
Some one who is a second cousin or closer should always show up on your match list. When two people are second cousins or closer, both of you will have inherited some identical segments of DNA from your shared ancestors and thus appear in each other’s match lists 100% of the time.
If a known second cousin (or closer) tells you they have taken a DNA test and they don’t show up, check you are both on the same database (not one on Ancestry and another on 23andMe for example) and that your settings allow you to see and be seen match wise. If this all checks out then you have a mystery to solve.
Once we are more distantly related the chances decrease.
Companies estimate that we won’t match up to 10% of 3rd cousins, up to 50% of 4th cousins and up to 90% of 5th cousins, although their statistics do vary (see https://isogg.org/wiki/Cousin_statistics) It is important here to understand the difference between possibilities and probabilities. There is always a possibility that you will match a cousin, even distantly related but the probability of doing so drops the further away the common ancestor is genetically.
Important point!
We get 50% of our DNA from Dad and 50% from Mom and so will each of our siblings. Due to recombination their 50% will contain some DNA that we also inherited but also some we didn’t – so if you don’t match a 4th cousin, your brother or sister might.
QUESTION 2
