Maternal blood iron level: For the first time, an external factor turns a male mammal into a female

https://english.elpais.com/science-tech/2025-06-04/for-the-first-time-an-external-factor-turns-a-male-mammal-into-a-female.html

For the first time, an external factor turns a male mammal into a female

An experiment in mice has discovered that a mother’s iron deficiency alters sex determination. Until now, it was believed that only genetics defined this biological trait

NUÑO DOMÍNGUEZ
JUN 04, 2025 - 12:39 EDT

All living people have been both male and female at the same time for six weeks. Although our sex is determined at the moment the sperm fertilizes the egg, it is not until a month and a half later that the biological programming to develop testes or ovaries is activated.

Textbooks have always taught that this process is 100% dictated by genetics: males have one female and one male chromosome, XY, and females have two female chromosomes, XX. But on Wednesday, the results of a bold experiment with mice were published, showing that very low iron levels in the mother can transform males into females, regardless of their genetics.

“To our knowledge, this is the first demonstration that an environmental factor can influence sex determination in a mammal,” Makoto Tachibana, a biologist at Osaka University in Japan and leader of the research, published in the journal Nature, tells EL PAÍS. “The most important implication of this finding,” he adds, “is that environmental and metabolic conditions influence fundamental developmental decisions that until now were thought to be strictly determined by genetics.”

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