New advancements in DNA technology over the last decade or so have given birth to an easier and more reliable way to match people and their relatives. But if you are someone who has taken a DNA test, you may have already discovered that you have 100s of cousins with various degrees of separation. So what do first, second, third and fourth cousins mean? Additionally, why do they have to be “once” or “twice” removed when you are building a family tree? In this article, I will break down the myths and facts of finding online DNA relatives when you receive your DNA results.
What are the chances of two people having the same DNA?
I think it is important to answer this question first as you will understand how DNA is shared between siblings and other members of the family. The short answer is that only some identical twins share the same DNA. You typically inherit 50% from each of your parents. You inherit mitochondrial DNA from your mother and this is how the mitochondrial haplogroup is determined. If you are a male, you will have the Y chromosome and this is inherited from your father. The DNA matches (in %) is calculated using autosomal DNA inherited from your parents. So as you can see from the below figure, the maximum you can inherit from your biological parents is 50%.
Recent studies have shown that even identical twins carry 10-15 mutations/variations. This suggests that even identical twins may not have the same DNA even though the same genetic information is passed on from their parents.
How much DNA do 4th cousins share?
Based on a study published by AncestryDNA if you have a British descent, you are likely to have an average of 193,000 cousins (5 first cousins, 28 second cousins, 175 third cousins, 1,570 fourth cousins, 17,300 fifth cousins, and 174,000 sixth cousins). Definitely, too many cousins! Here is another crazy data point: for an 8th cousin, this number can go upto 590,000.