The unique mutations in your brain and immune system too are induced during development. The body needs variation in neurons so they can recognize themselves m

The unique mutations in your brain and immune system too are induced during development. The body needs variation in neurons so they can recognize themselves millimeters away and not connect with themselves. And it needs immune cells that recognize a wider variety of proteins than could be encoded directly in DNA. So somehow mutations are induced in specific genome regions in the brain and immune system. Which isn't to say that mutations don't play a large role in other cells. The average child has at least one mutation but usually they don't do much. Well, doesn't do much or means the zygote is totally nonviable and so never results in a pregnancy. Having children when you're older will tend to increase the number of mutation, especially for men where their gametes are produced throughout their lives. A cell from, say, someone's skin will tend to have even more mutations but that's a quantitative thing rather than a qualitative one.