The final missing piece of the human genome has been decoded

Despite its macho connotations, the Y chromosome is among the tiniest of the 46 chromosomes in the human genome. It makes up only 2 percent of a human cell’s total DNA. But because of its seemingly endless repeating bases, the Y chromosome is one of the most difficult to genetically sequence. Scientists initially believed it was nothing more than a genetic wasteland, only good for making sperm.

Yet, in reality, that’s not the case at all. As genetic technology grows more advanced, so has our understanding of the Y chromosome’s importance. Its loss in older men, for example, is associated with an increased risk of cancer and other chronic diseases. Its genes somehow play a part in multiple biological processes. But, for decades, more than half of the Y chromosome remained unsequenced, and its role in human health remained a mystery.

That age of mystery is ending. For the first time, geneticists have assembled a complete sequence of the Y chromosome. The international Telomere-to-Telomere (T2T) Consortium added data for more than 30 million new base pairs and identified 41 new protein-coding genes. Two studies published today in Nature break down those findings, explaining how this chromosome affects our reproduction, evolution, and even the gut microbiome.

“The complete sequence of the Y chromosome has opened up a lot of doors for the scientific community,” says Chris Lau, a professor of medicine at the University of California, San Francisco who studies the human Y chromosome but was not involved in these current studies. “We anticipate some surprises could be forthcoming, just like the time in the past we thought it was full of junk materials.”

A picture a century in the making

It took more than 100 years for biologists to construct a complete assembly of the Y chromosome’s structure, after its discovery in 1905. The first human genome was completed in April 2003, but it left behind some unknown gaps, including swathes of the Y chromosome.

The chromosome’s repetition made it a challenge to reconstruct. It has more than a million of base pairs lined up in long re