The Y chromosome was the last of the 24 human chromosomes to receive a complete sequence, in August 2023. Its finishing marked a major milestone in the telomere-to-telomere effort to close the remaining gaps in the human reference genome.
The original Human Genome Project was declared complete in 2003, but that achievement covered roughly 92% of the genome and left difficult, repetitive regions unresolved. A near-complete genome without the Y chromosome was published in 2022. Researchers then used improved long-read sequencing and assembly methods to resolve the Y chromosome’s extensive repeated DNA.
The result added about 30 million base pairs to the previous reference and revealed additional genes and repeated structures. The Y chromosome is especially challenging because it contains large palindromic and tandemly repeated regions. This is why the answer is not chromosome 22, chromosome 8, or the X chromosome: those chromosomes were completed earlier in the modern finishing projects.