Scientists Create Fertile Mice from Two Male Parents Using DNA Editing

Scientists have spent decades steadily advancing research focused on producing mice from parents of the same sex. Now, genomics researchers from China have said that they successfully pulled off an experiment to produce mice from the DNA of two sperm cells.

While this scientific feat has been achieved before, the researchers also highlighted a key breakthrough: the motherless mice are not only healthy but also fertile, meaning they can go on to produce offspring of their own.

The study was published in Proceedings of the National Academy of Sciences (PNAS), a peer-reviewed scientific journal, on June 23, 2025, by researchers at Shanghai Jiao Tong University in China. The scientific demonstration involved the targeted editing of methylation using epigenetic programming, meaning that the changes do not affect the underlying DNA sequence.

The new research advances epigenetic programming as a potential reproductive tool that can be used to breed motherless mice without compromising on their health and fertility. It also validates certain theories about imprinting and embryonic development as well as the role of methylation in creating embryos.

While the efficiency of the epigenetic programming is still low, it is not zero either. “It confirms that genomic imprinting is the main barrier to uniparental reproduction in mammals and shows it can be overcome,” Dr Helen O’Neill, an associate professor and molecular geneticist at University College London, was quoted as saying in a report by New Scientist.

However, scientists do not believe that the epigenome-editing approach can be replicated in humans. “While this research on generating offspring from same-sex parents is promising, it is unthinkable to translate it to humans due to the large number of eggs required, the high number of surrogate women needed and the low success rate,” Christophe Galichet at the Sainsbury Wellcome Centre in the UK, was quoted as saying.

What is imprinting?

Mammals are known to chemically modify DNA through a process known as imprinting. While males imprint their DNA by adding methyl modifications in order to alter the activity of genes and promote the growth of embryos, females make chemical modifications to shut down certain genes for the same purpose.

There are seven critical imprinted regions scattered throughout the genome.

Having only the DNA modifications done by one sex was considered to be lethal as the embryo could not theoretically grow to match its stage of development. As a result, producing embryos using only the DNA from eggs or sperm cells was long thought to be impossible.

But roughly 20 years ago, a team of researchers successfully conducted an experiment in which a female mouse gave birth to offspring that had received a set of chromosomes each from just two unfertilised eggs – no sperm cells. The mouse with two mothers was named Kaguya.

By 2016, researchers could create embryos by editing the DNA to have deletions of imprinted genes and produce fatherless mice. Two years later, there was another breakthrough as researchers had managed to get the genomes of two sperm together in an unfertilised egg with its own genome eliminated, which led to the first motherless mice.

However, the offspring of the two male parents reportedly died one day after birth. Editing the DNA to put in deletions are said to have adversarial health impacts on the genetically modified offspring mice. Hence, epigenetic programming was not considered to be a reliable reproductive tool until this year.

How the experiment was conducted

As mentioned earlier, there are imprinting problems when both sets of chromosomes come from the same sex. The researchers behind the new paper have attempted to overcome this hurdle through the targeted reprogramming of methylation.

They used two distinctly related strains of mice for the experiment. One set of mice was from a lab in Europe while the other strain originated from wild mice in Thailand. The researchers used parts of the CRISPR and CAS DNA editing systems. They started with an unfertilised egg whose genome had been deleted. Then, they injected the egg with the heads of two sperm cells (one from the lab and the other from the wild mouse).

This ensured that the egg had two sets of XX and XY chromosomes. The YY chromosomes are inviable as X has essential genes, unlike the Y chromosome. The researchers arbitrarily labelled one set of chromosomes to be female and attempted to reprogramme the pattern of methylation through targeted editing. Once this was done, the egg was allowed to start dividing and subsequently implanted in the female mice to give birth.

Out of 250 reprogrammed embryos that carried DNA from two males, only 16 of them led to pregnancies. In addition, four more died at birth. However, there were three live births of male mice.

One of the three live ones died the day after birth as it was reportedly 40 per cent larger than a typical baby mouse.

Limitations of the study

Researchers have said that while properly reprogramming DNA at one imprinting site is possible, it is much more challenging to do it at all seven imprinting sites as there is the risk of off-target effects.

This means that the modification can take place in locations with DNA sequences similar to the ones that need to be targeted.

There is also the possibility of other key imprinted regions that have not yet been identified by researchers.

Previous research efforts

Earlier this year, another team of researchers from China were able to grow mice from two fathers. While these genetically modified mice were not fully healthy or fertile, they survived to adulthood after their genetic activity was normalised through 20 genetic modifications.

In 2023, a team in Japan said that they were able to produce mice with two fathers using a new technique that involves fusing stem cells into eggs.