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Y chromosome is smaller because it doesn't really contain as much genetic information as the X chromosome does.

X chromosome container several vital genetic information but Y has mostly genetic codes that are often redundant or obsolete and overridden by X except sexual development.

There are some animals like whiptail lizard which only have females because as they evolved their male allosome (the human equivalent of Y chromosome) became redundant and unnecessary.

See https://en.wikipedia.org/wiki/Sex-determining_region_Y_protein and https://en.wikipedia.org/wiki/Y_chromosome#Human_Y_chromosome

Most genes involved in male sexual characters are on other chromosomes but require a cascade initiated by expression of SRY (thus presence of Y chromosome) to be (un)expressed

Y chromosome in humans are getting shorter. It loses around 10 genes per million years. At this rate it will be gone in four and a half million years. Also sex-determination doesn't only involve the Y chromosome, but around 60 genes working in concert all over the genome.

You only want human facts? I've got some pretty interesting animal facts for the Y chromosome.

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The Y chromosome is unable to perform recombination so most damage is unable to be repaired. This causes loss of genetic information overtime, if the genes were critical to life the animal would be unable to reproduce. So only genetic losses that are no/low negatively impactful can be passed on.

> I've got some pretty interesting animal facts for the Y chromosome.

Let's hear them!

The Y chromosome is so much shorter than the X because there are much fewer genes & much less genetic information stored in that chromosome. This is because the Y chromosome can’t have any absolutely essential genes, because roughly half the population doesn’t have a Y chromosome and still needs to fully function & survive. The Y chromosome mostly contains genes related to triggering male development, but the genes that actually control male development are on many different chromosomes (including the X).

Everyone has at least one X chromosome, and in fact the X chromosome does contain some essential genes, which also speaks to its length.

> Also sex-determination doesn't only involve the Y chromosome, but around 60 genes working in concert all over the genome.

But aren't all of those triggered by SRY, which is (usually) located on the Y-chromosome?

It does have just genes for sex determination, so it does not need to be big because it carry not as much information as any of the other chromosomes.

You do not need two X chromosomes active, in fact, in women one of the X chromosomes is inactive and condensed in a Barr body.

I seem to recall learning somewhere that the male chromosome has been seen to disappear as you described, and then reevolved again later on in the same species (or perhaps one of its evolutionary offspring species so to speak). Is this true?

Sometimes SRY gets copied onto the end of an x chromosome during meiosis. People who inherit this end up as xx males.

SRY is like a conductor. It triggers the multiple genes to express testes or suppress ovaries. But those genes also trigger and suppress other genes. And while SRY is the trigger in some mammals, it's not necessary to kick start sex determination.

For example the platypus has five pairs of sex chromosomes. Females are XXXXXXXXXX and males are XXXXXYYYYY. Despite that many Y, on none of them SRY is found. One of those other interacting genes triggers sex determination. What's even more interesting those various involved genes can be found in similar combination throughout all vertebrae, but the trigger differs.

So while SRY is the trigger in some, it's not the only one.

I don't know about that. I'm referring to J. M. Graves, a professor of evolutionary genetics. She looked at how the platypus Y was different to the human Y and calculated how much genetic material had been lost since our species diverged. That's how she got that time frame.

But she also told that some (male) scientists aren't fans of this and try to prove how stable the Y chromosome is.

Hmmm, perhaps what I'm thinking is that multiple different male chromosomes have evolved in different branches of evolution?

I actually had to look some infos up again and found another fun (?) fact.

In South America there are nine species of vole (genus Akodon) in which a quarter of female are XY, not XX. Their Y chromosome is complete with SRY, yet they still develop ovaries and produce viable eggs. That suggests they must have a entirely new master switch gene that can suppress SRY.

TIL! Thanks :)

I didn't hear of something like that, but think it's possible.

If you remember or find the info it would be interesting to know.

Humans diverged from Macaques 25 million years ago and have lost one (1) gene on the y chromosome since then. The Y chromosome isn't going anywhere.

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I can't find this lady, what university does she work for?

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