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For decades, biology has observed an incontestable demographic fact: women live longer than men. It has often been blamed lifestyleto testosterone or to the greater male propensity for risky activities. However, science has found a much more subtle and genetic culprit that we carry in all our cells and that literally we start to lose as we get older. A genetics class. In a very general way, we must remember that all our genetic information is collected in 46 chromosomes which are found within the nuclei of our cells in pairs. But there is a part of all these chromosomes that define us as men or women: The presence of two X chromosomes defines women and the presence of one X chromosome with one Y defines men. Although there is great genetic complexity behind something as redundant as a pair of chromosomes, what interests us in this case is that science has seen a effect called mLOYwhich is literally the loss of Y chromosome mosaic in men. And different scientific articles suggest that it is not a simple side effect of getting older, but rather it is a “silent killer” that explains much of the longevity gap between the sexes. The runaway chromosome. For a long time, the Y chromosome was considered the “little brother” of the genome. Small, with few genes and almost exclusively responsible for determining the male sex with no other known functions, almost all of which fall on the X chromosome of considerable size. But the truth is that we were wrong, and the Y chromosome has great importance in the adult life of men. The mLOY phenomenon. This occurs when the cells that are in charge of manufacturing the blood elementslike erythrocytes, platelets, or lymphocytes, suffer errors when dividing and lose the Y chromosome. Something that generates a “mosaic” in our body, that is, some white blood cells have the Y chromosome while others do not. But what is disturbing is the frequency with which it occurs, since, according to the data reviewed, this is something that has been detected in 40% of men at age 60 and in 70% of men at age 90. There is damage. Until recently, it was believed that losing this chromosome was benign and normal, a simple “genetic gray hair.” But the evidence accumulated between 2022 and 2025, including massive UK Biobank studies and the recent German studio LURIChas set off alarm bells: losing the Y chromosome is not harmless and has important side effects. The heart. One of these side effects is precisely heart failure, which is a very prevalent disease in the elderly. Here science has been able to see that, by eliminating the Y chromosome in mice, the animals rapidly developed cardiac fibrosis. That is, their hearts were filled with scar tissue, becoming rigid and, therefore, having great difficulty pumping blood. But it is not the only disease that occurs, since in the United Kingdom Biobank, men with mLOY in more than 40% of their white blood cells had a 31% higher risk of dying from cardiovascular causes. And even the LURIC study published last year, carried out on 1,700 men, found that the mLOY effect increased the risk of fatal heart attack by almost 50%. More diseases. Beyond the heart, the impact of losing the Y chromosome also affects our body’s defense system to be able to combat different threats. Among them we have cancer, since the immune system needs the Y chromosome to effectively monitor the tumor cells that arise. Its loss is associated with a worse prognosis in bladder cancer and other solid tumors, since it is as if our body’s security guards had gone partially blind. In addition to cancer, the frequency of mLOY has also been seen to be up to 10 times higher in patients who have Alzheimer’swith studies showing an almost 3 times higher risk of developing the disease. The COVID. During the pandemic we saw that older men died much more than women without fully understanding why. We now know that the loss of the Y chromosome increases 54% risk of fatality for being infected with COVID in the elderly, finally offering a biological explanation for this bias. Is there a solution? It may seem depressing to know that a part of our DNA decides to abandon us and cause us so many problems, but in reality, it is a hopeful finding. And it is hopeful, since, seeing that the loss of the Y chromosome is a direct cause of a disease, therapeutic doors open. In experiments with mice, it has been seen that treatment with an antifibrotic drug was able to reverse the cardiac damage caused by the loss of the chromosome. This means that the mLOY effect can be used as a marker in a blood test, as happens with cholesterol, to predict a patient’s cardiac risk and to be able to give preventive treatments to delay it and improve the patient’s quality of life. Images | nrd Miroslaw Miras In Xataka | The X chromosome has new clues about aging: why women tend to live longer than men