Selfish chromosome almost wiped out humans millennia ago. It seems to have returned — in the Mormon state of Utah

Young missionaries of The Church of Jesus Christ of Latter-day Saints with the Book of Mormon. Illustrative photo.

In late February 2026, the pages of the authoritative journal Science featured an article by researchers Alexander Platte and Daniel Harris. The scientists tried to explain a well-known genetic paradox: in our X-chromosomes, traces of Neanderthal DNA are four to five times less than on other, regular chromosomes (autosomes). At the same time, Neanderthals themselves, on the contrary, show an excess of human DNA, which they inherited from even earlier contacts that occurred about 250,000 years ago.

The authors of the publication proposed a rather simple demographic explanation. In their opinion, such an imbalance arose because the interbreeding of the two species primarily followed one line:

Neanderthal men had relations with modern human women. Since the father only passes the Y-chromosome to his son, and the son always receives the X-chromosome from his mother, this marital imbalance supposedly led to a deficit of Neanderthal X-chromosomes in the modern population.

However, a number of evolutionary biologists consider this approach too simplistic. To reduce the proportion of Neanderthal DNA on the X-chromosome by as much as five times compared to autosomes, marital preferences or social structure alone are not enough.

A much more powerful mechanism was at play here — harsh natural selection, which literally purged Neanderthal genes from our genome due to their biological incompatibility in key survival processes.

The mechanics of the selfish chromosome

To understand the true reason for the purity of our X-chromosome, one must look at the process of germ cell creation (spermatogenesis).

Normally, X and Y-chromosomes have equal chances of being passed on to the offspring, ensuring a 50-50 sex ratio. But sometimes a phenomenon occurs in nature that biologists call "chromosomal drive" or intragenomic conflict.

Its essence is that as a result of a mutation, a selfish, toxic Y-chromosome appears. It begins to produce proteins that physically destroy or paralyze neighboring spermatozoa carrying the X-chromosome.

Since sperm precursor cells are connected to each other by cytoplasmic bridges, the toxin easily spreads and kills competitors.

A man with such a Y-chromosome begins to produce exclusively sons. For his personal genetic line, this is a short-term success, but for the population — a disaster, as with each generation the number of women rapidly decreases, leading to the extinction of the species.

Genetic data indicate that our ancestors encountered a similar toxic Y-chromosome shortly after leaving Africa, about 50-70 thousand years ago. Salvation came in the form of a new mutation on the X-chromosome, which acted as a molecular antidote against the Y-toxin. This protected X-chromosome began to spread rapidly, saving the sex balance. Traces of this powerful selection are still visible: about eleven percent of the length of the X-chromosome in modern inhabitants of Eurasia bears the imprints of that ancient genetic purge.

Molecular filter for Neanderthals

It is this mechanism that settles the question of why we have so few Neanderthal genes on the X-chromosome. When our ancestors met Neanderthals about 47,000 years ago, the evolutionary war between our X and Y-chromosomes was in full swing.

Павялічыць

Reconstruction of a Neanderthal man's appearance.

Neanderthal X-chromosomes, which entered the genome of hybrid children, simply did not have the necessary molecular antidote against the human selfish Y-chromosome.

As a result, hybrid men who received the Neanderthal X-chromosome faced serious fertility problems or produced sharply imbalanced offspring. Natural selection mercilessly eliminated such lines from the gene pool.

Neanderthal DNA was only able to establish itself in those parts of our X-chromosome that were not involved in protecting against chromosomal drive.

Therefore, today we see not the result of the romantic preferences of ancient humans, but the result of the action of a molecular filter that allowed only compatible genes to pass through.

Return of the selfish chromosome?

These molecular wars could be considered solely a matter of the prehistoric past, however, on February 23, 2026, a preprint of a study by scientists from the University of Utah appeared on the bioRxiv portal, bringing the problem into our time.

A team led by James Baldwin-Brown decided to check whether selfish chromosomes exist in modern humans. To do this, they used a unique resource — the demographic database of Utah Mormons, which contains information on more than 76,000 people with strictly documented pedigrees.

Павялічыць

Female and male sections of the famous Mormon Tabernacle Choir in Salt Lake City, the capital of Utah. If a selfish Y-chromosome has indeed appeared in the state, this gender balance could be significantly disrupted in subsequent generations.

Mormons (members of The Church of Jesus Christ of Latter-day Saints) keep detailed genealogies primarily due to their religious beliefs related to the salvation of ancestors and the eternality of families. Mormons believe that people who died without knowing their teachings should have a chance for salvation. Living members of the church perform the ordinance of baptism for their deceased relatives ("baptism for the dead") so that they in the afterlife can accept or reject this faith.

Thanks to this religious diligence, Mormons created FamilySearch — the world's largest genealogical database, housed in a special vault inside Granite Mountain (Utah, USA). These resources, which include scans of parish registers from Belarusian archives, are also open to people of other faiths who simply want to study their roots.

Using complex Bayesian algorithms, researchers analyzed birth statistics and identified one family whose demographics defy the laws of probability. Over seven generations in this male line, 60 boys and only 29 girls were born.

A two-to-one ratio over such a long distance and with such a sample size makes randomness practically impossible (statistical significance p=0.001).

Biologically, this is the most likely candidate for the role of a modern selfish Y-chromosome, which is currently, in the 21st century, conducting a molecular purge of X-spermatozoa in its carriers.

Of course, scientists leave room for social skepticism: since the first generations of this family lived in the 18th-19th centuries, the deficit of girls may be partly related to patriarchal negligence, where the birth of daughters was simply worse documented.

However, if further cytological analysis of the sperm of living descendants of this lineage confirms the presence of chromosomal drive, this will be the first proven case of such a genetic anomaly in modern humans.

• 9
• 1
• 1
• 4
• 6
• 0
• 0
• 0