According to the basic rules of genetics, a man passes down his genes to his child via sperm. These genes contain the same genetic code that was passed on to that man from his parents and to them from their parents. Or so we thought. A new study has found that a man's health — particularly around the time of conception — may directly affect the genes that are passed on to his offspring.

Take obesity, for example. Health experts once thought that a person was either genetically predisposed to obesity or not. But new research shows that a father's weight at the time of his baby's conception may change the genetic makeup of the DNA that he passes on to his child. And that change could in turn make his child more likely to be obese.

To tease out the answers, researchers first looked at the sperm from 10 obese men and 10 men of normal weight. They found several differences between the genetic markers found in the sperm of the obese men and those found in the sperm of the non-obese men. Specifically, they noted differences in genes that affect brain function and development — particularly as it relates to appetite control.

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But those could just be genetic differences that were in the genes of the obese men at birth, right? And those were passed down to them by their parents.

That's where the second phase of the study comes in. Researchers recruited six obese men who were planning to undergo bariatric surgery. These men donated three sperm samples — the first was donated a week before the surgery, the second the week after, and the third sample was donated one year later. Lead researcher Dr. Romain Barres and his colleagues from the University of Copenhagen found more than 4,000 genetic markers that were different one year after the men had surgery.

What's not clear is how these changes will affect a man's future children. “I don’t want to speculate whether it’s positive or negative in the following generation,” Barres said.

And it's not just a man's weight that can affect his sperm. Animal studies have found similar genetic changes in the sperm of male rats that were stressed at the time of collection.

These studies may change the way that geneticists look at sperm and DNA. What they show is that the genes that a man passes on to his child may not be set in stone at birth, but rather, they may change depending upon his health at the time of his child's conception. And while so much attention is paid to the health of the mother at conception and during pregnancy, these studies show that it may also be important to focus on the mental and physical health of a man who is hoping to become a father.