They are still trying to figure this out in science as it is still “unclear” what causes this “phenomenon”. Yet one ancient source has stated long ago, what it that is; “the two shall become one flesh.” (Mark 10:8, Gen. 2:24) Basically, God did it. The fact that anyone would (or could) come up with such a scientific assumption and be correct over 4000 years ago without any technology whatsoever is pretty dang miraculous.
More intriguing is the apparent fact that men do not absorb female DNA into their cells from their partners. As much as people hate to hear it this is exactly the picture of Judeo-Christian marriage where a man essentially ‘converts’ the woman by name to his own identity. She takes on his last name. It is not a unilateral change. Science is just discovering that she takes on his DNA also.