When Dutch authorities began investigating fertility doctor Jan Karbaat in 2017, the public's first reaction was disbelief: how could a physician secretly father nearly 200 children using his own sperm? But behind that question lurked a more troubling one — a question about genetics, probability, and a system failure hiding in plain sight.
Parents chose Karbaat for his appearance and credentials. Tall, fair-haired, medically educated — he embodied what fertility clinic clients often call "good genes." It looked like a guarantee. It turned out to be a trap.
Physical appearance, however, is just the genome's storefront. Behind it lie thousands of DNA variants invisible to the naked eye. After his death in 2017, genetic testing revealed that Karbaat carried a mutation in the TP53 gene — one of the most studied and most consequential genes in cancer biology.
The TP53 gene encodes a protein called p53, which biologists have dubbed the "guardian of the genome" — and the title is earned. Every second, hundreds of thousands of cell divisions occur in your body. Each division carries a chance of copying errors in DNA. The job of p53 is to detect damaged cells and either force them to repair themselves or trigger apoptosis — controlled, programmed cell death.
When TP53 is mutated, that surveillance weakens. This is why TP53 mutations are found in roughly 50% of all human cancers. The hereditary condition caused by an inherited TP53 mutation is called Li-Fraumeni syndrome, named for the two scientists who described it in 1969. Carriers face a lifetime cancer risk exceeding 90%.
A striking comparison: Elephants develop cancer at roughly one-twentieth the rate of humans — despite living far longer and having trillions more cells. The explanation, published in JAMA in 2015, turns out to be elegantly simple: elephants carry not two working copies of TP53, as humans do, but around twenty. Nature gave them a twentyfold safety margin. This discovery reshaped how scientists think about cancer suppression at the molecular level.
Here is where things get counterintuitive. Most people instinctively seek similarity in a partner or donor — in appearance, temperament, background. That impulse is understandable. But from a genetic standpoint, this strategy carries a risk that is rarely discussed.
Many dangerous mutations are recessive. This means that for a disease to manifest, a child must inherit a "broken" copy of the gene from both parents. If you carry one such copy, you are almost certainly healthy — your second, functional copy compensates. But if your partner carries the same mutation, the probability that your child inherits both broken versions is 25%.
This is why small, genetically isolated populations — historic villages, closed religious communities, groups with high rates of consanguineous marriage — show statistically higher rates of rare genetic diseases. Not because of "bad genes," but because of reduced genetic diversity.
In Karbaat's case, the scale of what he did created the same problem artificially. Nearly 200 children fathered by one man dramatically raised the probability that carriers of the same mutation would one day meet each other — as romantic partners. In the Netherlands, this scenario has already become both a legal and a medical reality.
Karbaat's story is not an anomaly. It is a symptom of a systemic failure in how sperm donation has been regulated. In most countries before the 2010s, the standard screening protocol for sperm donors included blood typing, a physical exam, and basic infectious disease tests. Genetic screening — expensive and technically demanding — remained optional.
That began to change only as DNA sequencing became cheaper. Today, whole-genome sequencing costs dozens of times less than it did in 2000. But regulatory standards move slower than technology: in most jurisdictions, expanded genetic screening for donors is still not mandatory.
If you are considering using donor genetic material — or if you are a donor, or were born from one — here are three things worth understanding:
In the paid article, we examine: how expanded genetic donor screening works today and which tests are genuinely necessary; why 90% of clinics still cut corners; what bioethics says about children's right to know about inherited risks; and how to verify whether a donor underwent NGS sequencing before you commit to a choice.
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