Fathers have been found to pass on four times as many genetic mutations to their children as mothers do. These tweaks to DNA, known as de novo mutations, are thought to be big contributors to rare childhood genetic diseases.

It turns out that – in Icelandic men at least – fathers pass on one new mutation for every 8 months of their lives, while mothers only do so every 3 years. This means that the majority of new mutations that children inherit come from the paternal side rather than the maternal.

The study, published in Nature, looked at the entire genomes of 14,000 Icelandic people. The Nordic island is perhaps the world’s greatest genetic laboratory, with genetic samples of around 150,000 people collected so far. This has allowed the researchers to study not only individuals, but also parents and their children, and sometimes even their children too.  

When humans create their sex cells, be it eggs or sperm, mistakes occur as the DNA is copied. These de novo mutations are changes in the genetic code that then go into these important cells. They do not affect the mother or father, but could potentially manifest themselves in their offspring.

Men tend to pass on more mutations simply because of the large volume of sperm they produce. The likelihood that mutations will occur also increases because men continue to make sperm over the course of their entire life. Women, on the other hand, are born with all the eggs they will ever produce – far fewer than the numbers of sperm produced by men.

The mutations are not necessarily a bad thing. After all, small changes to the genome are necessary for variation to develop within a population, and thus for evolution to happen. But occasionally these alterations can lead to the development of diseases, such as Tay-Sachs or cystic fibrosis.

“De novo or new mutations provide an important part of the substrate for evolution, launching a constant flow of new versions of the human genome into the environment,” explained Kari Stefansson, lead author of the study, in a statement. “However, they are also believed to be responsible for the majority of cases of rare diseases of childhood.”

“Providing a comprehensive catalog of such mutations from across an entire population is therefore not just scientifically interesting but also an important contribution to improving rare disease diagnostics.”

But there are some regions that seem particularly vulnerable to mutations passed on from the mother, such as a patch on chromosome 8. The researchers suspect that this part of the genome is particularly vulnerable to breaking and being patched up, hence why it has more mutations.