Young sperm, bad sperm

Schizophrenia, autism, spina bifida (open spine): Children will suffer from these illnesses their whole life. Chances to have them are significantly higher for children of young fathers, researchers have found. So young does not equal healthy – at least with regard to sperm. The likelihood of passing on genetic mutations that cause certain diseases are higher with fathers below the age of 19. Researchers have been aware of this fact for some years. So far though, nobody was able to explain it.

Paradoxical findings

"It seems paradoxical that boys have a higher rate of mutation than men aged 20 or above," says geneticist Dr. Peter Forster of Cambridge University. He believes he and his team have made a rather revolutionary discovery and can explain why teenage fathers are more likely to pass on genetically determined diseases.

As early as 2007, a US study testing five million babies and their parents showed that the highest risk category to pass on genetic mutations is that of fathers younger than 19 and older than 35 years. Men between 20 and 35 have the best sperm quality.

Higher mutation rate

Forster's findings are definite – and remarkable. He looked at a sample of 24,000 children and their parents, analyzing one genetic sequence that is passed on from parents to their descendents. "We were able to see a 30 percent higher rate in genetic mutations for young boys and their offspring, compared to the group of older males." It matches the rates of diseases caused by genetic mutations that earlier studies have found.

When DNA reproduces, errors can occur at certain points of the reproduction cycle. There is one step that is especially delicate. Just before the cell division, the DNA has to be copied. If the two strands of DNA miss their correct position, the genetic sequence becomes longer or shorter than it should. This can result in severe consequences, depending on the gene's function.

Flawed sperm production

But why would young males produce genetically defect sperm? Forster has two theories that might prove as an explanation. Either, he says, scientists have overlooked a long series of cell divisions in young boys, even before the onset of puberty. These could result in genetic mutations that are then reproduced and passed on. "The other reason could be that sperm production in young males is just not well-rehearsed. It might take a few years for the body to get good at it and make as little mistakes as possible."

Forster does not claim to have found the final explanation for the higher rate of genetic mutation in young men's sperm. "There will always be factors that influence the occurrence of illnesses which we cannot measure with any scientific instrument. However, we have found a strong correlation between the male's age and his rate of genetic mutations," Forster says.

To the geneticist, it is important to put his study in perspective. 1.5 percent of children by fathers aged 20 to 35 have defects caused by genetic mutation. "Citing our study, young males under 19 have a 30 percent higher chance of genetic mutations. This means that in the end, two percent of the high risk category children that have been fathered by males under 19 will suffer from genetic mutations. I want to stress, though, that 98% of the children from this category are healthy human beings."