As you have probably gathered, I am not a geneticist. I am a genealogist so I will try to explain in the best layman’s terms as I can about DNA you inherit from your ancestors.
I like the way Ancestry DNA explain the randomness of DNA Inheritance.
Looking at the top picture below, assume each letter in the blocks below represents a segment of DNA. Examining the blocks, you can see how different segments of DNA were passed down from Glenda’s grandparents to make her unique DNA. Notice how the amount of DNA she received from a particular ancestor decreases over generations and how her DNA differs from that of her siblings.
Another good way of explaining DNA inheritance is by focusing on your immediate family. If your parents’ genes were each a deck of 52 cards, you’d receive exactly 26 cards from each of them to form your own deck of 52. If you had a younger sibling, he or she would also receive 26 cards from each parent, and (due to probability) about half of that sibling’s 52 cards would end up being the same cards you received. If a third sibling were born, that sibling would share about 26 cards with each of you as well. This pattern would continue with each sibling born. Due to the random nature of inheritance, some siblings inherit more of some ethnicities and traits than others do, and some siblings may not inherit certain ethnicities or traits at all. Looking at my own family, it’s easy to see which siblings look more like my mother or father or indeed by maternal grandmother (I’m supposed to look like her).
As a general rule of thumb, this is what DNA inheritance looks like;
Your parents: You share about 50% of the same DNA of each of your parents – as do your siblings.
Aunts and Uncles: You share about 25% of the same DNA
Your grandchildren and grandparents; You share about 25% of the same DNA
Your 1st cousins: You share 12.5% of the same DNA
Your 1st cousins once removed: You share 6.25% of the same DNA
Your 2nd cousins: Your share 3.125% of the same DNA
In the next DNA post, I will talk about how many relatives the average family has and how many it is possible to find.