DNA Test: Why Siblings Can Get Different Results from DNA Ancestry Tests

You are probably surprised to hear that siblings—even twins—can have different DNA test results on ancestry tests.

Like with genes, many people would think that their ancestry and living relatives would all have very similar—if not the same—DNA results. For example, you and your siblings get a DNA kit through the mail for DNA or ancestry testing. You then provide a DNA sample via a cheek swab and saliva sample and then that sample is sent to one of the many DNA testing services available. However, after the testing of raw data for DNA and genetic data is complete and you get the results, it may surprise you. You may even think that the DNA testing company sent the wrong results of the DNA matches as your DNA is quite different from those of your relatives.

However, it is nothing to do with the testing companies, your relatives doing the swab incorrectly, or the DNA testing itself. It all comes down to cell division, the process of your cells multiplying, and its effects on your DNA. So, let’s talk about cell division first before we get into how it is possible that siblings who come from the same parents can have different results on DNA testing.

Each cell multiplies (or divides). The process is known as cell division and the sequence of events that take place are part of something called the cell cycle. Also taking place during cell division is DNA replication or creation of more copies of DNA. Overall, one cell gives rise to two cells—also known as daughter cells—each of which contains half the DNA that the previous cell had. This is confusing, yes. But hang on just a few sentences more.

There are three distinct types of cell division, binary fission, mitosis, and meiosis. Binary fission is cell division among bacterial cells. Mitosis and meiosis are cell division processes among non-bacterial species, including humans. For the purposes of this post, we will focus on meiosis only. In short, meiosis is a process during which a cell goes through two divisions to produce a total of four daughter cells. The cells that undergo meiosis are human sex cells—sperm in males and eggs in females. The first division is called meiosis I and the second is called meiosis II. After each division, the number of cells that are produced doubled. So, the process begins with one cell that goes through meiosis I to form to daughter cells. Each of the two daughter cells undergoes meiosis II to form two daughter cells themselves. This results in a total of four daughter cells that are produced. Furthermore, meiosis is the body’s way of producing human sex cells, also called gametes. The overall process of meiosis has a total of nine stages that are equally distributed between the first cell division (meiosis I) and the second one (meiosis II).

So, what happens to the genetic material during all of this and how does it relate back to siblings and their DNA test results?

Meiosis I begins at a stage called Interphase

In order for that to become clearer, let’s get into the nine stages a little bit. Meiosis I begins at a stage called Interphase, during which DNA is copied into two full sets of identical chromosomes. These chromosomes then condense into X-shaped structures and pair up during the next stage, Prophase I. At the completion of Prophase I, the copies of chromosome 1 are paired up and the copies of chromosome 2 are together, and so on. The next stage, Metaphase I is marked by alignment, whereby the chromosome pairs line up along the center of the cell. These chromosome pairs are then pulled apart, or separated so that each chromosome half travels to the opposite pole of the cell. This happens during Anaphase I. The last phase of this round is called Telophase I, whereby the chromosomes at each pole of the cell come together and become surrounded by a membrane. Two nuclei are also formed in preparation for cell division. Ultimately, the parent cell creates two daughter cells during a process called cytokinesis.

Essentially, something similar happens during the next set of phases, Prophase II, Metaphase II, Anaphase II, and Telophase II followed by cytokinesis. The only difference is that at the end of the first division, the daughter cells have 2N the number of genetic material and are called diploid, while each daughter cell at the end of the second division has half that, or N, and are called haploid.

So, let’s get back to Prophase I for a little bit. To the point where the chromosomes pair up so that the two copies of chromosome 1 are next to each other and the two chromosome 2 copies are positioned similarly. It is at this point that these pairs of chromosomes can exchange bits and piece of DNA during a process that is called recombination or crossing over. Literally, these chromosomes “swap” DNA regions or these cross over from one chromosome to another. And this is exactly why siblings may get different DNA test results—it is the process of recombination that is responsible for inter-people differences, even among those who are related.

The best way to think about the process of recombination or crossing over is to imagine each chromosome as a piece of rope. Imagine you have four pieces of rope, each piece representing one half of a chromosome, two blue ropes and two red ones. If you place them on a flat surface and you simply cross one red rope over a piece of blue rope, you will have an X-shaped structure. It is at this stage that little pieces of DNA on the chromosome come loose and travel over to the other chromosome. So, imagine that after crossing a blue rope and a red one, pieces of the red one break off and become part of the blue one and vice versa. That is exactly how DNA recombination takes place. Of course, there is elegant biology behind the process, but this is essentially how it happens.

To get back to our siblings and their DNA tests, this phenomenon of two individuals with the same mother and father having different DNA test results makes a lot more sense now. Crossing over or recombination does not happen at the same places and at the same times in each individual. It is a unique process, which is why we are all as different as we are and which is also why siblings have varying results on DNA tests.

Laura Day