Scientists made mice with Y chromosomes female by deleting just 6 tiny molecules

MaMMAls' chromosomes have a huge iNFLuence over whether an individual develops as male or female — but a new study reveals that the pull of these sex chromosomes can be overridden by tiny molecules called microRNAs. 

The study, published May 7 in the journal Nature Communications, showed that deleting the genes behind specific microRNAs could transform male mice into females in the womb, sparking a complete sex reversal.

"We did not expect that the results would be as spectacular as they are," study co-author Rafael Jiménez, a professor of genetics at the University of Granada, told Live Science. 

Sex determination in mammals relies on a fine balance between "opposite" sets of genes — one that drives the development of female characteristics, such as ovaries, and another that produces male characteristics, such as testes. Early in an animal's development, the scales tip one way or another, leading to an irreversible cascade of steps that ends in the development of either set of sex organs.

"In a very early stage of our development, all the mammals have the capacity of being male or female, potentially," said study co-author Francisco Barrionuevo, a professor of genetics at the University of Granada.

Related: The human Y chromosome has finally been fully sequenced, 20 years after the 1st draft

A gene called SRY, which is only found on the Y chromosome, triggers the series of events that forms testes. The gene's absence in individuals with only X chromosomes results in the formation of ovaries. Scientists know a great deal about the genes involved in making the proteins needed for these processes. But a huge portion of mammals' DNA — including about 98% of the human genome — doesn't code for any proteins, so scientists were unsure what role these other genes play in sex determination, if any.