Family Tree and DNA Inheritance

"Pleased to meet you cuz!"

Family TreeWe have a lot of ancestors if you think about it. We have 2 parents, 4 grandparents, 8 great grandparents, 16 great, great grandparents and so on. If you go back 10 generations (approx. 250 years) we would each expect to have over one thousand direct ancestors (one thousand 8xgreat grandparents). If you go back 500 years to around the time of Henry the VIII each of us could theoretically have had up to around a million direct ancestors living at that time. So, for example, if your ancestry is largely from the British Isles it would not be in the least bit surprising if you had a direct bloodline to Henry the VIII (or at least to his parents as his own children did not produce any offspring!).

If you take this logic to its inevitable conclusion, it is certain that all humans alive today anywhere in the world are related to each other via common ancestors. Some very esteemed genetic genealogists, taking into account a host of historical information about the movement and interbreeding of human populations have convincingly estimated that a common ancestor to every single person on the planet today lived only around three or four thousand years ago. So, you can see, if we go back far enough we are all related to each other no matter what our geographic or racial origins… “pleased to meet you cuz!”.

An important consequence of the existence of a multitude of forebears, especially if we are trying to find relatives that are alive today, is that we all have many more second cousins than we do first cousins, even more third cousins and a huge number of 4thcousins etc.

Different Kinds Of DNA That Can Be Tested
There are three kinds of DNA that are routinely tested which are informative in different ways and are best used to answer different questions depending on what the purpose of the test is. For example, is the test being used to confirm an existing relationship with, say, someone you suspect is a cousin or distant cousin or is it more about trying to find relatives that you are not yet aware of and have never met?

The three kinds of DNA we test are called; autosomal DNA, mitochondrial DNA (mtDNA) and Y-chromosome DNA. The vast majority of the DNA we possess is autosomal and we inherit an even mixture of it from all of our direct forebears both male and female. Y-chromosome DNA is only present in males and is only inherited along the unbroken male line. MtDNA is an extra small loop of DNA found outside of the chromosomes that we all have but it is only passed on from our mothers through the unbroken female line, males have it but don’t pass it on. See the diagram below.

Diagram of DNA InheritanceInheritance of DNA. Y-Chromosome DNA is passed unchanged from father to all male children along the unbroken male line (blue). Mitochondrial DNA is passed unchanged from mother to daughter along the unbroken female line (pink). Males receive mtDNA from their mothers but don’t pass it on to their children. Autosomal DNA is inherited evenly from all of our ancestors (blue, pink and white).

The vast majority of the DNA that makes us what and who we are is autosomal DNA and it makes up 22 of our 23 chromosome pairs. The 23rd, non-autosomal chromosome, is the male Y-chromosome, the presence or absence of which determines gender. In males it is paired with the X-chromosome giving (X,Y) and it is absent in females; who have a pair of X-chromosomes (X,X). Unlike the DNA within the Y-chromosome, we all inherit an equal and even mixture of our autosomal DNA from all of our direct forebears. We have around 50% of our mothers and 50% fathers autosomal DNA, 25% each of our 4 maternal and paternal grandparents DNA, 12.5% each of our 8 great grandparents DNA and so on. With Y-chromosome DNA we (males) share 100% of it with everyone in our unbroken male line going back through the generations and zero% of it with all other ancestors. The inheritance pattern for mtDNA is exactly equivalent to Y-DNA except in the female line.

Now look through the other pages in this section to see how testing these different types of DNA can be used to explore your family tree and find new relatives.





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