A new study from researchers at the University of Oxford has essentially taken genetic snapshots of the sperm in the testicles of 14 men of various ages. The results give us a clearer picture of why the children of older dads are at a higher risk for genetic diseases like schizophrenia, autism, dwarfism and even some late-onset cancers.
It has been known for a while that the older a man gets, the more genetic differences or mutations crop up in his sperm's DNA. This is because men make sperm at a furious rate, with sperm cells dividing constantly.
As new sperm cells are made, their DNA needs to be copied. And while astonishingly accurate, the machinery is not perfect, resulting in an occasional mistake.
What the new study helps to explain is why some of these mistakes are more common than others. It turns out that natural selection of sperm in the testicle plays a big role in at least some of these errors.
It’s a Jungle in There
Inside the testicle there is a set of stem cells that eventually become sperm. These are the cells that need to keep dividing to produce the 1,500 sperm that the average male makes each second.
After these cells split, they need to grow big enough to divide again. This is where the trouble can start.
If one of these stem cells happens to pick up a mutation that makes it grow a bit faster than the other stem cells around it, then that stem cell will become more common over time. It will divide faster and outcompete its non-mutated brethren.
This seems to be what is happening in a slew of rarer genetic conditions like dwarfism, and syndromes like Apert, Crouzon, and Pfeiffer. Each is caused by a mutation in a gene that makes the stem cell divide more quickly.
So this is one reason these mutations are more common in the sperm of older men. The longer the mutated stem cell has to grow and divide, the more of them there will be.
In other words: the older the man, the higher the concentration of mutant stem cells in his testicles, the study found.
And the more of these mutated sperm cells there are, the higher the risk that one will eventually fertilize an egg.
Men Are From Mars
It is important to note that all men pass on mutated DNA to their kids. It is just that older men pass on DNA with more mutations.
This is very different from women. They too pass on mutated DNA, but the amount doesn’t really change much with age. A 20-year-old mother and a 40-year-old mother will pass on the same number of DNA changes.
This is because while sperm are constantly being made, eggs are not. A woman is most likely born with all the eggs she will ever have.
This means the eggs of a 20-year-old mom have undergone the same amount of DNA copying as that of a 40-year-old.
Now of course that is not to say the children of older moms are not at higher risk for genetic problems. In older mothers, an extra or missing chromosome can lead to genetic conditions like Down syndrome or Turner syndrome.