Two Peas in a (family) Pod?

“My brother has already taken a DNA test. Do I need to take one as well? We have the same parents. Wouldn’t my DNA be the same as his?” Stacey F., submitted to the “Raise Your Hand!” section of DNAClassoom.com

This question comes up quite often, but just as two peas in a pod might be quite similar, they’re not exactly the same. Even though full siblings will have a lot of similar DNA, each child will have unique DNA that the other does not share. Let’s explore how that happens . . . Each of us receives 50% of our DNA from our fathers and 50% from our mothers. But, the 50% of DNA we were given does not represent each parent’s full DNA. We also did NOT inherit 50% of each parent’s DNA. Some of the DNA that was not passed to us, may have been received by a full sibling. And, that’s where the differences happen. To understand how this works, let’s remember that each of us has two pairs of each chromosome to equal our 46 chromosomes.

1. Source Citation: National Library of Medicine

Chromosomes 1 - 22 are known as the Autosomal Chromosomes, whereas chromosome 23 determines our biological sex, XX for a female and XY for a male. XDNA and YDNA will be discussed in other Lesson Blogs. Thus: Child from father: Autosomal chromosomes 1 - 22 Child from mother: Autosomal chromosomes 1 - 22 Tip! Just as our DNA is in pairs, our parents’ DNA is ALSO in pairs. Therefore, the DNA being passed down doesn’t just represent our parents but, more accurately, it represents our four grandparents. To visualize this, we might illustrate it with a chromosome segment in 4 different colors to represent each of our four grandparents.

When your father passes any particular segment of DNA to you, he can only pass EITHER his father’s DNA or his mother’s DNA in that particular segment. The same thing happens with DNA segments passed to us from our mothers. Through this process, the child’s paternal chromosome will usually undergo a process called “Recombination” and the paternal chromosome the child inherited will then contain DNA from both paternal grandparents. The same thing happens with the DNA inherited from our mothers. Sometimes, however, recombination does not occur, and the child will retain a complete copy of that parent’s paternal or maternal chromosome, thereby receiving none of the DNA on that chromosome from the other grandparent. Let’s explore how Recombination might play out in two siblings on the same segment of paternal DNA: 1. Child 1 and child 2 both receive DNA from their paternal grandfather - a match.

2. One child receives the DNA from the paternal grandfather while the other child receives the DNA from the paternal grandmother - not a match. Where opposite DNA is inherited, two siblings will not match one another.

While both children will have inherited DNA from all four grandparents, they may be in different locations and represent different distant ancestors. As you begin to think of your DNA as coming from various distant ancestors, ancestors that may be different than what a sibling inherited, you will also begin to understand why full siblings may also have different ethnicity results as well. To illustrate this further, let's use an example of two grandchildren compared with their mother and their maternal grandmother. Using the free sharing site, GEDmatch Genesis (source: 2), we can look at each of these examples and draw some theories about the DNA inheritance each of these children received from their maternal grandparents.

Mother to Children. Source 2: GEDmatch Genesis.

This first image is a comparison of the mother and her children. You can see that, with the exception of the low information region of the centromere, that these children match their mother from end to end on chromosome 12. We cannot tell from this image whether the individual segments contained here are from the maternal grandfather or the maternal grandmother. Since the maternal grandmother has tested, we can begin to see how their mother passed down DNA from this grandmother to her children.

Maternal Grandmother to grandchildren. Source 2: GEDmatch Genesis

In this example, the grandson shown on the top has inherited significantly more DNA from his maternal grandmother than has the granddaughter, shown on the bottom. ( Note: In this example, the colors of the segments are unimportant to this discussion as they merely represent categories of segment size.) What is important is that you can see that on this particular chromosome, the son and daughter only have one single shared segment. GEDmatch Genesis tells us that this overlapping segment is from position 71 - 96 for 21.2 centiMorgans (cMs). Note that the granddaughter shares an additional segment towards the end of chromosome 12 that the son did not receive. But, what about all of that DNA that the mother and her children share in the image above? If we had available data from their maternal grandfather, you would see those "missing" segments. Since the maternal grandfather is deceased and was never tested, we can use DNA from other close relatives of the paternal grandfather to fill in some blanks. In the following image, we compare the children to their paternal granduncle - the half-brother of their grandfather who shares their paternal great-grandfather.

Paternal grand-uncle to grand-niece and grand-nephew. Source 2: GEDmatch Genesis

In this image, the daughter is now the match on the top. You can see that space at the end where she matches her maternal grandmother in the image above. But, what about the other gaps that are not filled in between the maternal grandmother and the paternal half-uncle? Without the grandfather tested, we cannot tell if those gaps represent DNA from the paternal grandmother that the half-uncle does not share, or if it is different paternal grandfather DNA that the two brothers do not share. More research is needed before drawing reasonable conclusions about which more distant ancestors contributed to each of these children. And this is why two peas from the same family pod may be similar, but they are not exactly the same! Source Citations: 1. Illustration: National Library of Medicine (US). Genetics Home Reference [Internet]. Bethesda (MD): The Library; 2019 Apr 2. [Illustration] Chromosomes 1-22, X, and Y; [cited 2019 Apr 13]; Available from: https://ghr.nlm.nih.gov/gallery?start=20 2. Images: GEDmatch Genesis. Image capture, chromosome 12, various. {cited 2019 Apr 16]; https://genesis.gedmatch.com #SiblingDNA #DNAInheritance #understandingDNAResults #DNApainting #chromosomemapping