One way of detecting mutations is to sequence the genomes of parents and their offspring. In this example, 78 family trios were surveyed for mutations, and these are the results:
| Type of mutation | N | Rate per base per generation |
|---|---|---|
| Transition at non-CpG | 2489 | 6.18×10−9 |
| Transition at CpG | 855 | 1.12×10−7 |
| Transversion at non-CpG | 1516 | 3.76×10−9 |
| Transversion at CpG | 73 | 9.59×10−9 |
| All | 4933 | 1.20×10−8 |
First off, we can see that transitions at non-CpG sites is about twice that of transversions at non-CpG sites for rate per base per generation (the column on the right). Since CpG’s are very susceptible to mutation they are counted separately. CpG transition mutations are about 20 times more common (on a per base basis) than transitions at non-CpG sites. As you will notice, there are two figures: number and rate. Even though CpG transitions have the highest rate there are relatively few CpG’s in the genome compared to single A’s or T’s, for example. This is why we need to look at both the number and the rate.
As we would expect from our knowledge of biochemistry, transition mutations should outnumber transversion mutations and CpG transition mutations happen at the highest rate. This is observed in living populations.
Next, we will use these observations to test hypotheses about common descent and how evolution explains the differences between species.