Does My Cousin Get a Say in Whether I Have a DNA Test?

The National Human Genome Research Institute says that we humans share 99.9 percent of our genetic makeup. Of the other 0.1 percent, you and a first cousin will share, on average, 12.5 percent of your DNA (with a range, according to 23andMe, from 4 percent to 23 percent). Because what DNA you share is a result of random processes, it won’t be possible to tell much about her genetic makeup from yours. The fact that you have genes associated with some health condition, for instance, doesn’t mean that she does.