Hi . A thought occurred to me the other night while viewing the Project , where they discussed the extinction of men as the Y chromosome is generationally experiencing damage. I gathered at some point in human reproduction that the replication of the Y chromosome will become impossible.

They proposed that humans need to develop a way to compensate or we will become extinct.

I wondered whether that evolutionary change was already active with XXY possibilities. It seems that female and male bodies can form with this anomoly. Could this capacity be how our bodies will adjust to the shift or is does this still present with the same challenges?

I understand that fertility is affected with the XXY but are there any documented cases so far where a viable and naturally conceived pregnancy has occurred?

What advantage or disadvantage could being born with XXY and whatever other combinations could have in the future when considering the Y dilemma for males?

As I understand it (though I don’t claim specialised knowledge), the role of the Y-chromosome is little more than a trigger for forming male-specific characteristics, and that most of the genes for actually forming those characteristics are on the X-chromosome (a defect in the trigger gene leads to an genetically XY individual who is phenotypically female, though I don’t know how this affects fertility).

The logic of sexual reproduction is that two individuals, from each of the two genders, provide haploid gametes which combine to form an individual that is one of the two genders. In humans, females can only supply X-chromosomes, while males can supply either an X-chromosome or a Y-chromosome. Thus, when the gametes from a male and a female are combined, there are only two possibilities: XX (female) and XY (male). Note that if females could supply both X and Y* (where the trigger gene is inactive), then there would be four combinations: XX (female), XY (male), XY* (female), and YY* (unviable).

The non-existence of the Y-chromosome is logically equivalent to the existence of the Y-chromosome in that XX (female) can mate with X0 (hypothetically male, but actual X0 individuals are female) to produce either XX or X0 (the 0-gametes are simply missing a chromosome). However, because genes tend to be additive in effect, the manifestation of X0 as male would require somehow that a pair of X-chromosomes act distinctly different to a single X-chromosome (to change X0 from currently female to male).

On the other hand, one could simply have a single gender which mates with another member of that gender, perhaps as a hermaphrodite, or as an enviromentally-determined gender, to produce an individual of that gender (XX + XX –› XX).

> the Y chromosome is generationally experiencing damage.

It has the greatest damage rate of any chromosome, and there’s good reason for that. Because it has the lowest density of genes of any chromosome. So it is least affected by an equivalent amount of damage. Damage on other chromosomes is much more severe.

> I wondered whether that evolutionary change was already active with XXY possibilities. It seems that female and male bodies can form with this anomaly.

I’ve said before that a large number of people are unaware that they have extra chromosomes because they’re phenotypically normal. This doesn’t just affect the sex chromosomes, people pick up small “marker” chromosomes as well. The average number of human chromosomes is not 46, it’s more like 46.005 or even more.

KJW said:

As I understand it (though I don’t claim specialised knowledge), the role of the Y-chromosome is little more than a trigger for forming male-specific characteristics. However, because genes tend to be additive in effect, the manifestation of X0 as male would require somehow that a pair of X-chromosomes act distinctly different to a single X-chromosome (to change X0 from currently female to male).

I assume you’re talking about the SRY gene
Wikipedia said:

SRY (Sex-determining region Y) is a sex-determining gene on the Y chromosome in the therians (placental mammals and marsupials).

This intronless gene encodes a transcription factor that is a member of the SOX (SRY-like box) gene family of DNA-binding proteins. This protein is the therian testis determining factor (TDF), referred to as the sex-determining region Y protein or SRY protein which initiates male sex determination. Mutations in this gene give rise to XY females with gonadal dysgenesis (Swyer syndrome); translocation of part of the Y chromosome containing this gene to the X chromosome causes XX male syndrome.

KJW said:

On the other hand, one could simply have a single gender which mates with another member of that gender, perhaps as a hermaphrodite, or as an enviromentally-determined gender, to produce an individual of that gender (XX + XX –› XX).

Don’t forget about X-inactivation
Wikipedia said:

X-inactivation (also called lyonization) is a process by which one of the two copies of the X chromosome present in female mammals is inactivated. The inactive X chromosome is silenced by it being packaged in such a way that it has a transcriptionally inactive structure called heterochromatin. As female mammals have two X chromosomes, X-inactivation causes them not to have twice as many X chromosome gene products as males, which only possess a single copy of the X chromosome (see dosage compensation). The choice of which X chromosome will be inactivated is random in placental mammals such as mice and humans, but once an X chromosome is inactivated it will remain inactive throughout the lifetime of the cell and its descendants in the organism. Unlike the random X-inactivation in placental mammals, inactivation in marsupials applies exclusively to the paternally derived X chromosome.

PM 2Ring said:

Don’t forget about X-inactivation

For the most part, I wasn’t actually talking about genetic abnormalities, but was talking about the logic of sexual reproduction with regards to maintaining it in spite of the loss of the Y-chromosome. In the case of X-inactivation, or the case of the X-chromosome absorbing the SRY gene, they are both logically equivalent to the hypothetical X0 case I mentioned (and to each other). Thus, we could have:

XX (female) + XX* (male) –› XX (female) + XX* (male)

where X* is either the inactive or Y-absorbed X-chromosome. This also assumes that the male and female gametes are functionally complementary. Otherwise, we could have:

XX* (male) + XX* (male) –› XX (female) + XX* (male) + X*X* (???)

KJW said:

This also assumes that the male and female gametes are functionally complementary. Otherwise, we could have:

XX* (male) + XX* (male) –› XX (female) + XX* (male) + X*X* (???)

In this thread, I’ve assumed two genders, either genetically distinguished, environmentally induced, or in the case of hermaphrodites, at the gamete level. But, what would be the consequence of more than two genders? Complementarity of gamete function implies two genders, so what would be the functional relationship between (say) three types of gametes? What physical manifestation of the gametes would support such a functional relationship?

From http://www.news-medical.net/health/Y-Chromosome-Evolution.aspx

In the terminal stages of the degeneration of the Y chromosome, other chromosomes increasingly take over genes and functions formerly associated with it. Finally, the Y chromosome disappears entirely, and a new sex-determining system arises. Several species of rodent in the sister families Muridae and Cricetidae have reached these stages, in the following ways:

The Transcaucasian mole vole, ‘‘Ellobius lutescens’‘, the Zaisan mole vole, ‘‘Ellobius tancrei’‘, and the Japanese spinous country rats ‘‘Tokudaia osimensis’‘ and ‘‘Tokudaia muenninki’‘, have lost the Y chromosome and SRY entirely. ‘‘Tokudaia’‘ spp. have relocated some other genes ancestrally present on the Y chromosome to the X chromosome. Both genders of ‘‘Tokudaia’‘ spp. and ‘‘Ellobius lutescens’‘ have an XO genotype, whereas all ‘‘Ellobius tancrei’‘ possess an XX genotype. The new sex-determining system for these rodents remains unclear. The wood lemming ‘‘Myopus schisticolor’‘, the arctic lemming, ‘‘Dicrostonyx torquatus’‘, and multiple species in the grass mouse genus ‘‘Akodon’‘ have evolved fertile females who possess the genotype generally coding for males, XY, in addition to the ancestral XX female, through a variety of modifications to the X and Y chromosomes. In the creeping vole, ‘‘Microtus oregoni’‘, the females, with just one X chromosome each, produce X gametes only, and the males, XY, produce Y gametes, or gametes devoid of any sex chromosome, through nondisjunction.

Outside of the rodent family, the black muntjac, ‘‘Muntiacus crinifrons’‘, evolved new X and Y chromosomes through fusions of the ancestral sex chromosomes and autosomes. Primate Y chromosomes, including in humans, have degenerated so much that primates will also evolve new sex determination systems relatively soon, in about 14 million years in humans.

PM 2Ring said:

Don’t forget about X-inactivation

KJW said:

For the most part, I wasn’t actually talking about genetic abnormalities,

Neither was I: X-inactivation is a normal thing; female mammalian cells with both X chromosomes active are abnormal.

KJW said:

but was talking about the logic of sexual reproduction with regards to maintaining it in spite of the loss of the Y-chromosome. In the case of X-inactivation, or the case of the X-chromosome absorbing the SRY gene, they are both logically equivalent to the hypothetical X0 case I mentioned (and to each other).

Understood.

The X chromosome can absorb the SRY gene, so it’s possible for the SRY gene to survive even if the Y chromosome gets junked. Of course, an X chromosome containing an SRY gene (or part thereof) is currently considered an abnormality.

PM 2Ring said:

The X chromosome can absorb the SRY gene, so it’s possible for the SRY gene to survive even if the Y chromosome gets junked. Of course, an X chromosome containing an SRY gene (or part thereof) is currently considered an abnormality.

If the X chromosome does absorb the functionality of the Y-chromosome, such that:

XX (female) + XX* (male) –› XX (female) + XX* (male)

where X* is the Y-absorbed X-chromosome, then this will be a good thing for the male gender, making them genetically more robust. Currently, males have only one copy of the genes found on the X-chromosome, many of which do not pertain to sexual function, and therefore lack the redundancy of having two copies, leading to condititions such as colour-blindness which are much more common in males than in females. Absorbing the Y-chromosome into the X-chromosome would provide males with the desired redundancy.

Men need more than just SRY from their Y-chromosomes – one can appear a normal male (phenotypically) as XX+SRY but won’t be fertile. For sperm production, multiple other genes from the Y chromosome are required.

Interestingly, IVF is increasing the number of subfertile males. Some men need to use IVF bc they have abnormal sperm resulting from Y chromosome gene mutations, and using IVF means their sons are inheriting the same mutation.