chromosomes determine sex or the other way around - proof?

[C-Talos]

Hi folks,

 

In a cool MIT lecture (Genetics 2), Eric Lander says the following (quote from the transcription):

 

Males have an X and Y. Does it have to mean that they determine gender? Maybe gender determines them. Maybe what happens is that in female cells you get both chromosomes, but in male cells some enzyme comes along and chews off the end of the chromosome. No, no, really. Maybe this is some physiological state of the chromosomes. Why are you so ready to leap to the conclusion that the chromosomes determine sex, rather the gender, than the gender determines the chromosomes?

 

Good question. He then goes on to describe the Morgan lab results about eye colour being linked to sex. Then he says:

 

Thus, the transmission of eye color, a trait controlled by a gene on the X chromosome correlated so beautifully with the transmission of the trait sex. That provided a convincing argument that it was the chromosomes controlling sex rather than the sex controlling chromosomes.

 

I don't quite get this. It's still just a correlation, right? If you asked before "hey perhaps sex (caused by some unknown factor F) affects the chromosomes after fertilization?" then you can now ask "hey perhaps factor F affects the chromosomes AND consequently the eyecolour they determine, after fertilization?".

 

I'm obviously not really disputing the chromosome->trait causality. This is about the historical methodology issue that Lander raises; and I don't quite get the answer. Is it ultimately just a question of not finding good candidates for this hypothetical factor F, whereas the chromosome as the causal agent seems very plausible? In other words: a probability and parsimony-of-explanation argument rather than a logical proof? Any insights?

 

Thanks!

[Paralith]

There are more differences between the X and Y than just length. Actual sequences are different. The Y chromosome contains SRY, the male sex determining gene, that initiates the signal cascade that causes the undifferentiated genital ridge to be come testes. The X chromosome does not have this gene.

[pioneer]

From just practical logic, the female side of reproduction is far more complicated. It not only has to provide eggs but it is also involved in gestation, hormonal changes, nurturing and maternal behavior which allows a woman to relates more to the baby. All that extra implies we need more genes for the sex based tasks. Even if we knew nothing of particular genetic function, common sense would say X is female and Y male due to the extra leg of genes gives more genes for all the extra tasks.

 

Humans have free will, which means we can make choices with instinct or outside of instinct. It is not within genetic extinct to vomit out food. But free will can detach from instinct and genetics. The free will is a wild card variable that is not limited to genetics, especially when behavior is involved. Willpower can create the illusion you can get more genetic behavior with less genes. It can create a genetic version of perpetual motion. This is why it is important to upgrade to logic since empirical can use indirect evidence to support perpetual motion.

[Mr Skeptic]

Another problem with that theory is that then your sperm bearing the Y chromosome would be, well, dunno. Does he also hypothesize a way to turn Y chromosomes back into X chromosomes?

 

Also, XX vs XY is only one way sex is determined. And in some creatures, such as limpets, the sex can be changed back and forth due to environmental conditions. And some like earthworms are both male and female at the same time.

[C-Talos]

Thanks for your thoughts people!

 

Mr Skeptic, you are right, he would have to make that assumption. This 'unknown sex-determining factor F' would have to modify a Y into an X if female, and an X into a Y if male. As Paralith notes, this would involve more than 'chewing off' bits or 'pasting them on'. This scenario may seem far-fetched or inefficient, but not therefore impossible?

 

So let's assume that these geneticists around 1910 were thinking that might be the case. Then I still don't see how this business of sex-linked genes such as white in Drosophila would have convinced them that it is not the case.

 

Perhaps I'm not clearly understanding how much information geneticists had back then. I'm assuming they could clearly observe chromosomes in somatic cells and gametes. A bit of observation should then have suggested the actual "X/Y chromosomes determine sex" hypothesis as a likely one, but still leaving the more convoluted one as an option...?

 

BTW, pioneer, I'm a bit skeptical about your 'female chromosome is larger because female reproduction is more complicated' theory. Even if, in line with your theory, it is also the more bulky chromosomal package that says 'female' in the sex determination system in birds. But check this page: "The X chromosome carries hundreds of genes but few, if any, of these have anything to do directly with sex."

[Mr Skeptic]

The X chromosome has little to do with sex, other than that it will replace the Y chromosome. Female is the "default" body plan, but the genes on the Y chromosome change it to male. Women have two X chromosomes but will (randomly) deactivate one or the other X chromosome in each of their cells, so that they do not get a double dose of the genes on it.

[SkepticLance]

One strong clue to cause and effect is timing. Cause precedes effect. If, in the genesis of a human, the chromosome precedes the sex development, the chromosome is the cause. If sexual development precedes, then it is the cause.

 

As we all know, sperm and eggs bear chromosomes. The sperm either has X or Y. Until sperm and egg unite, and for a long time after, there is no gender. The chromosome came first, so it is the cause, and gender, which develops later, is the effect.

[C-Talos]

to SkepticLance: But in the alternative pathway involving unknown factor F, X&Y-chromosomes would still precede sex development - the difference is that they themselves would be preceded and modified by the (largely invisible - perhaps it sits on an autosome) factor F. This alternative still agrees with the known timing which you mention.

 

I suppose no one has ever noticed an X from a gamete turning into an Y after fertilization, or the other way around, so there's certainly a lack of positive evidence for such a pathway...

 

But if I understand Eric Lander's lecture there was indeed an issue, and I'm trying to understand this issue and how the sex-linked genes provided a resolution.

 

thanks again both!