The Incredible Shrinking Chromosome

A pair of equals

About 180 million years ago, in a small shrew-like mammal living alongside the dinosaurs, the ancestors of today's X and Y were an ordinary matched pair of autosomes. Same size, same genes, and like every other pair, they lined up at meiosis and swapped segments by crossing over.

Then, on one copy, a duplicate of the gene SOX3 mutated into something new: SRY, a switch that tells the developing gonad to become a testis. Whichever copy carried SRY was now, in effect, a proto-Y. Comparisons across mammal genomes date this to roughly 180 million years ago, in the common ancestor of marsupials and placental mammals. The platypus and echidna, which branched off earlier, have a completely different set of sex chromosomes with no SRY at all.

Why the Y shrank: life without a partner

Crossing over is how a chromosome repairs itself over evolutionary time. If one copy picks up a damaging mutation, recombination can pair it with a clean copy and let selection sort the good combinations from the bad. To keep SRY and the male-specific genes near it together, the proto-Y gradually stopped crossing over with the X. It did this in blocks, most likely through inversions on the Y, and each block left a signature on the X that geneticists call an evolutionary stratum.

Once a block stopped recombining, two well-understood processes went to work on the Y copy:

• Muller's ratchet. Think of photocopying a photocopy. In a population of Y chromosomes that never recombine, the copies with the fewest mutations get lost by chance now and then, and they can never be rebuilt. Each loss is one click of the ratchet.
• Hitchhiking and background selection. On a chromosome that never recombines, every gene shares one fate. A helpful mutation can drag harmful ones along with it as it spreads, and selection against harmful mutations drags down the good genes linked to them.

The X did not suffer the same fate because in females it still pairs and recombines with another X. One more twist added material: roughly 80 to 130 million years ago, in the placental ancestor, a block from another chromosome was added to the short arms of both X and Y. Most of today's Xp comes from that X-added region, and most of it then went through the same shutdown on the Y (strata 3 to 5).

The damage, by the numbers

Note what the 3% figure measures: genes, not DNA. The Y is still more than a third the length of the X because much of it is repetitive sequence, including the huge heterochromatic block at Yq12 and the ampliconic regions full of testis-specific gene families. The first complete, telomere-to-telomere sequence of a human Y, published in 2023, added more than 30 million bases that earlier references could not assemble. The Y has sometimes been called a genetic wasteland, but the genes it kept are anything but junk.

Seeing it at the bench: the Y on a karyotype

A routine karyotype usually starts with peripheral blood. Lymphocytes are stimulated to divide, arrested in metaphase, and the chromosomes are G-banded (trypsin, then Giemsa stain). They are then arranged by size, centromere position and banding pattern into 22 autosome pairs, with the sex chromosomes at the end.

The size gap between X and Y is obvious, but the Y is not a speck. It is about the size of the G-group chromosomes (21 and 22) and, like them, acrocentric, though it has no satellites. A few features worth knowing:

• Yq12 varies from man to man. The distal long arm is heterochromatin, and its size is a normal inherited variant (written Yqh+ or Yqh-). It fluoresces brightly with quinacrine (Q-banding) and is a classic source of "is this Y abnormal?" questions.
• SRY sits right next to PAR1. SRY is at Yp11.2, just inside the pseudoautosomal boundary. An abnormal crossover in the father's meiosis can carry SRY onto the X, which is the usual cause of 46,XX testicular disorder of sex development (the "XX male"). FISH with an SRY probe is the standard way to show it.
• Sex chromosome aneuploidies. Klinefelter syndrome is 47,XXY; Turner syndrome is 45,X (often written "XO" in older genetics texts, but 45,X in ISCN). A person with 47,XXY develops as male because the Y, and SRY, is present.

The plot twist: losing the Y in a single lifetime

The Y is not only disappearing over evolutionary time. As men age, some of their dividing cells drop the Y entirely, usually through a mis-segregation at cell division. The result is mosaic loss of Y (mLOY): a mix of normal 46,XY cells and 45,X cells that have lost it.

mLOY is the most common acquired chromosome change in human blood. With sensitive genotyping it is detectable in about one in five men in the UK Biobank, and it becomes more common with every decade of age. For years it was dismissed as a harmless quirk, since the Y carries so few genes. That view has changed:

• mLOY in blood is associated with shorter survival and higher risk of non-blood cancers, and with Alzheimer's disease.
• In mice, blood cells lacking the Y drive heart fibrosis, and in the UK Biobank mLOY is associated with death from heart failure. That is some of the first evidence that the loss can cause disease, not just accompany it.
• Environmental exposures speed it up. Smoking is strongly and dose-dependently associated with mLOY, and the effect appears partly reversible after quitting. Arsenic in drinking water has also been linked to it, in a study of Bangladeshi men.

This is one of the most common findings in karyotypes from older men. It is often written up as an abnormal result, and technically it is a clonal change, yet on its own it is usually a sign of age rather than of a blood cancer.

Will the Y disappear completely?

In 2002, geneticists Robert John Aitken and Jennifer Graves did a back-of-the-envelope calculation. If the Y had lost genes at an average rate of about 4.6 per million years, and only a few dozen were left, a straight-line extrapolation said the last of them would be gone in roughly 10 million years. (The "4.6 million years" figure that circulates online is that rate, misread as a deadline.) Graves herself has stressed it was never meant as a prediction.

It is also not a far-fetched idea. Some rodents have already lost the Y completely. Two species of mole vole (Ellobius lutescens and E. tancrei) have no Y and no SRY, and their new sex-determining gene is still unknown. The Amami spiny rat of Japan also lost its Y; there, males are set apart by a small male-specific duplication near the gene SOX9 on an ordinary chromosome, which appears to have taken over SRY's job.

But the evidence from our own lineage points to stability:

• The losses stopped. Comparing human and rhesus macaque Y chromosomes shows almost no gene loss since the two lineages split about 25 million years ago: one gene lost on the human side and none on the macaque side. The decay was fast early and then leveled off.
• What is left is essential. The surviving ancestral genes are dosage-sensitive regulators, expressed widely across the body, which is why selection holds on to them.
• The Y repairs itself with palindromes. Much of the male-specific Y is organized into eight large palindromes, sequences that read the same forward and backward on opposite strands, with arms up to about 1.45 Mb long and 99.97% identical. The two arms can exchange sequence (gene conversion), so a mutation on one arm can be corrected from the other. It is a kind of recombination the Y does with itself.

The bottom line

The Y is a shadow of the chromosome it once was. It lost around 97% of its ancestral genes, and it keeps losing ground in individual men as they age. But the collapse was front-loaded, the genes that remain are hard to live without, and the chromosome has its own repair system. The Y is small, but it is not in a hurry to leave, and SRY, the gene that started it all, has held its post for about 180 million years.

Frequently asked questions

Sources

1. Cortez D, et al. Origins and functional evolution of Y chromosomes across mammals. Nature 2014;508:488-493.
2. Lahn BT, Page DC. Four evolutionary strata on the human X chromosome. Science 1999;286:964-967.
3. Ross MT, et al. The DNA sequence of the human X chromosome. Nature 2005;434:325-337.
4. Bellott DW, et al. Mammalian Y chromosomes retain widely expressed dosage-sensitive regulators. Nature 2014;508:494-499.
5. Hughes JF, et al. Strict evolutionary conservation followed rapid gene loss on human and rhesus Y chromosomes. Nature 2012;483:82-86.
6. Hughes JF, et al. Chimpanzee and human Y chromosomes are remarkably divergent in structure and gene content. Nature 2010;463:536-539.
7. Rhie A, et al. The complete sequence of a human Y chromosome. Nature 2023;621:344-354.
8. Rozen S, et al. Abundant gene conversion between arms of palindromes in human and ape Y chromosomes. Nature 2003;423:873-876.
9. Aitken RJ, Marshall Graves JA. The future of sex. Nature 2002;415:963.
10. Forsberg LA, et al. Mosaic loss of chromosome Y in peripheral blood is associated with shorter survival and higher risk of cancer. Nature Genetics 2014;46:624-628.
11. Dumanski JP, et al. Smoking is associated with mosaic loss of chromosome Y. Science 2015;347:81-83.
12. Dumanski JP, et al. Mosaic loss of chromosome Y in blood is associated with Alzheimer disease. American Journal of Human Genetics 2016;98:1208-1219.
13. Thompson DJ, et al. Genetic predisposition to mosaic Y chromosome loss in blood. Nature 2019;575:652-657.
14. Sano S, et al. Hematopoietic loss of Y chromosome leads to cardiac fibrosis and heart failure mortality. Science 2022;377:292-297.
15. Somatic loss of the Y chromosome is associated with arsenic exposure among Bangladeshi men. International Journal of Epidemiology 2023;52(4):1035. Link
16. Just W, et al. Absence of Sry in species of the vole Ellobius. Nature Genetics 1995;11:117-118.
17. Terao M, et al. Turnover of mammal sex chromosomes in the Sry-deficient Amami spiny rat is due to male-specific upregulation of Sox9. PNAS 2022;119:e2211574119.
18. Pečnerová P, et al. Genome-based sexing provides clues about behavior and social structure in the woolly mammoth. Current Biology 2017;27:3505-3510.