How many brand-new mutations is each baby born with?
Every baby carries dozens of changes in its DNA that neither parent had, and most of them come from the father's side.
▶ Start the storyA human genome accumulates around 64 new mutations per generation, according to one estimate, although estimates of the exact rate have varied by an order of magnitude or more. These are de novo mutations: changes newly present in an individual, arising in a parent's sperm or egg cells or after fertilization. The human genome mutation rate is estimated at about 1.1 × 10−8 per site per generation.
The rate is higher in the male germ line than in the female. The reason is cell division. Each year after puberty, the precursors of sperm divide about 23 times, so by age 40 they will have gone through about 660 divisions, compared to 200 at age 20. Each division means copying the DNA, and copying errors may lead to new mutations in sperm DNA.
Sperm-precursor divisions by father's age
divisions
| Divisions of spermatogonia | |
|---|---|
| Age 20 | 200 divisions |
| Age 40 | 660 divisions |
What do these changes do? It is now generally thought that most mutations are mildly deleterious, many have little effect on fitness and a few can be favorable. A mutation in a parent's sperm or egg can cause a genetic disorder in a child even though neither parent shows any sign of it.
The risk to any one child from an older father is small. A 2017 review found that the total increase in problems caused by paternal age is low at an individual level, yet statistically significant across a population.
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Recap
Each DNA copying can have a typo, and sperm precursors divide about 23 times each year after puberty.
💡 A trick to remember it · Every baby is a fresh photocopy, and the more times a master gets copied, the more smudges sneak in.
Surprising fact · A human genome accumulates around 64 new mutations per generation, more of them from the male germ line.
Connects to
- 📖 How did humanity read its own genome, and what did it find?
- 🎛️ What is epigenetics, and how much of the hype is true?
- 🧬 Where do brand-new traits come from in the first place?
- 🧬 Why does the risk of cancer increase as you get older?
- 🧬 How does a four-letter DNA alphabet spell out a protein?
- 🦋 How does natural selection actually change a species?
Sources (3)
No source, no claim. Every fact in this lesson (15 claims) cites at least one of these.