Home / Blog / Sperm donors, hidden half-siblings and inherited genes

Sperm donors, hidden half-siblings
and the genes nobody checked.

One sperm donor can have hundreds of children, and most of them never meet. That raises two questions most families never think to ask: could two of them fall for each other, and what else did he pass on?

By the DontMarryYourBro team · Updated October 2026 · 7 min read · How we fact-check

The short answer

  • One European donor with a rare cancer-causing gene variant fathered at least 197 children. He had passed the bank's screening.
  • A Dutch court found another donor had likely fathered 550 to 600 children, and ordered him to stop in 2023.
  • The US has no national legal limit on children per donor. Colorado is the first state to cap it, at 25 families.
  • Half-siblings share about 25% of their DNA. A DNA comparison spots them instantly. A family tree can't.

The donor who passed every test

In 2005, a student became a sperm donor at the European Sperm Bank in Denmark. He was healthy and passed the bank's screening. His sperm was sold to 67 fertility clinics in 14 countries.

Years later, doctors found that some of the children carried a rare change in the TP53 gene, which causes Li-Fraumeni syndrome, a condition that raises the lifetime risk of cancer dramatically. The donor himself was healthy. The variant was found in only part of his sperm, up to about 20% of it, which helps explain why routine donor screening didn't catch it.

In December 2025, an investigation by 14 European public broadcasters, including the BBC, found the donor had fathered at least 197 children. Of 67 children identified earlier, 23 carried the variant, and 10 of them had already been diagnosed with cancer. Some have died. In Belgium alone, 53 children were born to 38 women from this one donor, in a country that limits each donor to six families.

The man with hundreds of children

In April 2023, a court in The Hague ordered a Dutch donor, Jonathan Meijer, to stop donating after finding he had likely fathered between 550 and 600 children through clinics and private arrangements in several countries. Dutch guidelines allow 25 children in 12 families. The court found he had misled prospective parents about how many children he already had. His story became the Netflix series The Man with 1000 Kids.

How many children can one donor have?

  • United States: no national legal limit. The American Society for Reproductive Medicine notes it has been suggested that donors be limited to about 25 births per population of 800,000, but that's guidance, not law, and tracking is patchy.
  • Colorado: the first state to set a limit. Its 2022 law caps each donor at 25 families and ends anonymous donation, for donations from 2025.
  • Europe: limits differ by country. In July 2026, the European Society of Human Reproduction and Embryology (ESHRE) called for a Europe-wide cap of 50 families per donor, moving toward 15.

Risk 1: falling for a half-sibling

When one donor has dozens or hundreds of children, often born around the same years and in the same region, the chance that two of them meet goes up. Most donor-conceived people know nothing about their donor siblings, and many don't know they're donor-conceived at all.

Half-siblings share about 25% of their DNA, around 1,759 cM. Marriage between half-siblings is illegal in every US state, and children of half-siblings face a higher risk of recessive conditions than children of first cousins. You can't see a half-sibling in a family tree. You can see one instantly in DNA. How much DNA siblings share.

Risk 2: what the donor passed on

Donors are screened, usually for infectious diseases and for carrier status of common recessive conditions. But screening has limits:

  • It can't catch everything. The TP53 case shows a variant can hide in part of a donor's sperm and pass every test.
  • One variant can reach hundreds of families. Whatever a mega-donor carries, his children's children may carry too.
  • Carrier matching matters. If the donor and the recipient both carry the same recessive condition, each pregnancy has a 1 in 4 chance of it. That's why recipients should be carrier screened too. How two healthy parents pass on a condition.

If you're donor-conceived

  • Take a consumer DNA test such as AncestryDNA or 23andMe. Half-siblings who've tested appear in your match list. Which kit to choose.
  • Join the Donor Sibling Registry with your donor number, if you know it, to find others conceived from the same donor.
  • Before things get serious with someone who is also donor-conceived, or adopted, compare DNA. Our free relatedness check compares two raw DNA files in your browser, nothing uploaded.
  • Before planning a family, get a carrier screen. Your donor's carrier results may be incomplete or out of date.

If you're using a donor

  • Ask how many families the donor has been used for, and what the bank's limit is.
  • Ask for the donor's carrier screening results, and get carrier screened yourself, so the two can be compared.
  • Keep the donor number and records. Your child may need them for health reasons, or simply to know.
  • Report any genetic diagnosis in your child back to the bank, so other families can be told.

General information, not medical or legal advice.

Sources

  1. CBS News: Sperm donor with cancer-causing gene fathered nearly 200 children across Europe (December 10, 2025)
  2. STAT: 200 children inherit gene mutation from one sperm donor (December 10, 2025)
  3. Al Jazeera: Sperm donor who fathered at least 550 kids banned by Dutch court (April 29, 2023)
  4. Colorado General Assembly: SB22-224, Donor-conceived Persons and Families Protection Act
  5. MIT Technology Review: Sperm donors need limits, says a European fertility group (July 10, 2026)
  6. Shared cM Project v4 (DNA Painter)

Donor-conceived? Check before it's serious.

Compare two raw DNA files in your browser and see if you're related, half-siblings included. Nothing is uploaded. Free while in beta.

Check if we're related

Questions people ask.

How many children can a sperm donor have?

In the US there is no national legal limit. Colorado caps donors at 25 families for donations from 2025. In Europe limits vary by country, and ESHRE recommended a Europe-wide cap of 50 families per donor in 2026. Some donors have been found to have hundreds of children.

Can donor-conceived half-siblings accidentally date?

It is possible, especially when one donor has many children born in the same region and years. Half-siblings share about 25% of their DNA, so a DNA comparison shows the relationship immediately.

Are sperm donors screened for genetic diseases?

Usually for infectious diseases and carrier status of common recessive conditions. Screening cannot catch everything: in one European case, a cancer-causing TP53 variant was found in part of a healthy donor's sperm, and he had passed screening.

How do I find my donor siblings?

Take a consumer DNA test such as AncestryDNA or 23andMe and check your match list, and register with the Donor Sibling Registry using your donor number if you know it.

Should I get carrier screening if I used a donor?

Yes. If you and the donor carry the same recessive condition, each pregnancy has a 1 in 4 chance of it. Comparing your carrier results with the donor's is the way to check.