Tell me about DNA and how it works
We inherit half of our DNA from each parent, always. That’s how it works at conception.
Imagine two people, Tom and Carol. They have two babies together called Joseph and Lucy and each passes on 50% of their DNA to each child. So, the children are made up of 50% DNA from Tom and 50% from Carol. But of course, Tom and Carol’s own DNA is actually what they inherited from their own parents in the same way. So now we understand that Joseph and Lucy actually carry around half of Tom’s father’s and half of Tom’s mother’s DNA, and the same again with Carol’s mother and father. And it follows that those grandparents have received their DNA from their own parents, so in truth Joseph and Lucy have actually inherited an eighth of their DNA from each great grandparent. And so on…
The next thing to grasp is that the inheritance of 50% of their DNA from each parent is random. Imagine Tom’s DNA is a giant bowl of smarties in two colours; half are red ones representing his father, and half are green from his mother. At their conception, Joseph and Lucy received a random half of those smarties from him. The law of averages would suggest that about half of what they received were red and half were green, but they might actually have got slightly more of one colour than the other. And they will have picked up some identical smarties but also some different ones. Now imagine that repeated with Carol’s DNA which is blue and yellow smarties.
Perhaps Joseph has more green than red, more blue than yellow. And Lucy could have more blue too but less green.
In other words, Joseph and Lucy, (unless they happen to be identical twins), do not share exactly the same mix of DNA from their parents. The smarties in their bowls are different. That is because everyone has their own unique mix of DNA, inherited differently.
Numerically, Joseph and Lucy will share with each other between a third and a half of identical DNA (where they got the same smarties) and the rest will be different (they got the other smarties).
In future Lucy will have children of her own, and those children will inherit half of their DNA from her. Some of it will be from Tom and some from Carol, again randomly. Those children will share approximately a quarter of their DNA with each of their grandparents, but it won’t be a precise amount. They will also inherit 50% of their DNA from Bob who is Lucy’s partner. For this generation, Bob’s smarties now enter the ‘bowl’, new colours representing his own parents.
As each generation passes, new smarties will be introduced, and a random half of the bowl will be passed on. Eventually, in generations to come, there will be none of Tom and Lucy’s smarties left, they will have been reduced randomly by approximately 50% each time until they have all gone.
For those of you who are keeping up, we believe the actual extent of autosomal DNA is somewhere around six to eight generations. So, it is reasonable to say that the average person’s DNA is made up of very approximately the collective DNA of our five-times great-grandparents, and it spans around 250 years.
How DNA is calculated and then reported in test results – centimorgans and segments
Wherever you take your DNA test, the results should always include a list of relatives with whom you share some identical DNA. And they will be listed in order of how close a family member they are to you. Parents and children are always at the top of the list, if they have tested, followed by immediate family, then close family and then a whole bunch of cousins. The results will detail precisely how many smarties, which are officially called centimorgans (cM) you share with each of the people on your list. We inherit approximately 3500cM from each parent, so we have around 7000cM altogether.
Although the smarties explanation works well descriptively, DNA doesn’t actually take that shape, when it is passed on. Really DNA is made up of chromosomes. Leaving aside the chromosome that determines whether you will be born male or female, there are 22 in total. They each take different shapes, and at conception, each one splits into little pieces or segments. Half of those segments recombine with half of the segments from the other parent’s 22 chromosomes to make 22 entirely new chromosomes. For this part of the explanation, it would be more useful to think of DNA as 22 strips of dried spaghetti made up of broken segments, glued back together.
With most testing laboratories, your results will usually say that you share x cMs of DNA spread across x segments with someone on your list. Let’s say you share 100cM across 7 segments. This means that you inherited 7 bits of DNA from the same ancestors as they did. There may have been a piece that was 12cM long, one at 8cM, one at 25cM, and so on. And added together there were 100cM. We are often asked if there is any significance to the number of segments, we share with a DNA match (someone on our list), and truly there isn’t. It is the overall total of cMs that matters, rather than the number of segments it is made up from. However, for your interest, it is generally the case that bigger segments are better than smaller ones. If you think back to the spaghetti being regularly broken and put back together, you can see that some of the segments of the original stick will become very small over time. The bigger segments have generally come from more recent ancestors, and they haven’t been subjected to as many generations of divide and reform.
The other thing it would be useful to point out is that DNA genealogists discount any segment of less than 7cM in length (in other words less than 0.001% of your total DNA) because it is too small to be reliable. So, when we are counting segments that two people have in common, there may be a few that are only perhaps 5cM, but we set those to one side and don’t count them in the total, they are probably just “noise”.
How much DNA do we share with different types of relatives?
There is a handy table which shows how much DNA we share on average (the white chart). You can see that you always share 50% of your DNA with each parent (and child) – around 3500cM (3485 on this chart). The next closest relationship is your full sibling (where you have the same two parents), you will share between a third and a half. You could share between 1613cM and 3488cM, but the average is 2613cM. After this comes half siblings (where you share only one parent) along with aunts/uncles, nieces/nephews, grandparents/grandchildren. This will be around a quarter of your DNA or an average of 1750cM.
Once you look further than these close family members, into the realms of cousins, the amount will decrease with each generation in either direction.
We also recommend that you use the DNA Shared Painter Tool which will tell you about the most likely relationship between any two individuals based on how much DNA they share.
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