Scientists have fully sequenced the Y chromosome for the first time, making it easier to study conditions and disorders such as lack of sperm production that leads to infertility. “This is really a huge shift in what’s possible,” said Monika Cechova, co-lead author and postdoctoral scholar in biomolecular engineering at the University of California, Santa Cruz. Humans typically have a pair of sex chromosomes in each cell -- an X and a Y in those assigned male at birth and two X chromosomes in those assigned female at birth. The completed Y chromosome adds more than 30 million base pairs to the human reference genome. Earlier this year, researchers compiled a “pangenome” that incorporates the DNA of dozens of people from nearly every continent -- updating the human genome to make it more equitable and inclusive. In accompanying research, scientists assembled Y chromosomes from 43 male individuals from 21 world populations to capture genetic variation in the Y chromosome.