Every person has two copies of a human genome, one inherited from each parent. Before a parent passes down a genome to a child, the parent's two copies of a genome crossover and recombine with each other in a random process. One of these copies is randomly chosen and passed to the child as shown in Figure 1. (Be aware, these illustrations are a simplification of the process; the genome is billions of letters and typically there are dozens of crossovers.)
DNA inherited by siblings
The random crossover that occurs when passing down DNA from parent to child is different every time, and the chromosome that is passed to a child is completely random. So, your brother or sister will have different DNA than you do.
For example, if a sister and brother take a DNA test, the siblings will have long stretches of DNA letters in common. We call each of these segments a "DNA match" (see Figure 2). Sometimes the siblings match on the blue and red sections of their chromosomes, sometimes just on blue, sometimes just on red.
DNA inherited by cousins
Now, let's look at how DNA would be passed down to first cousins. Figure 3 shows DNA matching between two first cousins. First cousins (who share a pair of grandparents and have no other common ancestors) can only match on one of their two chromosomes. It's important to notice that first cousins share less DNA than the pair of siblings. The amount of DNA that individuals share is what helps us to estimate their relationship.
In figure 4, you can see how DNA is passed down to distant cousins. All three cousins descended from two common ancestors. But for these distant cousins to have a DNA match, it is not enough that they each inherit some DNA from a common ancestor, they have to inherit the same DNA. Cousins C and D are a DNA match and so are cousins D and E. However, cousins C and E are not a DNA match—based solely on DNA, they would not appear to be related.
As you can see, after many generations only a little DNA from a common ancestor is passed on. In fact, since the recombination process exchanges entire sections of DNA, and the pieces of DNA you inherit are random, you may not have inherited any DNA from some of your ancestors. Table 1 shows how likely it is that you will inherit any DNA from a distant ancestor. The more generations that separate you and the ancestor, the more likely it is that you did not inherit any of their DNA.
|Generations removed||Likelihood of inherited DNA|
For you to be a DNA match with someone, the same section of DNA would have to be passed down to both of you. Since DNA recombination is a random process, there may be no common DNA inherited by both of you. Table 2 shows how likely you are to have a DNA match with a cousin.
|Relationship||Likelihood of a DNA Match|