genetics

[phospho]

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I was going through some random genetics notes for M1, and I seem to be confusing myself about something. This is what I had written down a few months ago in my notes:

"Over 99% of expressed phenotypes need both homologous chromosomes to be expressed. That means that Prader–Willi and Angelman Syndromes would be part of the 1%, since the malfunction occurs because only one chromosome is required, and that required chromosome happens to be dysfunctional. It's worth noting that this might explain why we can't do parthenogenesis"

Now, I just remembered that while studying for the MCAT there was the Lyon's hypothesis which talked about X-inactivation in females, and how females can't have the 2 X's working simultaneously.

Do you think there's something wrong with my notes?

So males have nothing inactivated?

If females are getting their X inactivated, is the X chromosome in females part of that 1% I mentioned above in my notes?

Sorry if I'm confusing things together. I really really really suck in genetics, and tying facts together sometimes helps me keep a grip on things.

thank you for your help! 🙂

 

[RogueUnicorn]

X is one of the rare exceptions, by virtue of the fact that there necessarily has to be an asymmetry between males and females. girls have 2, dudes have 1. as you know from karyotype pictures, the Y chromosome is tiny - it essentially just codes for dude parts. since X actually codes for somatic genes unlike Y, we need to make up this difference somehow, or women and men would be much different - either a doubling of expression of X in males (a strategy in some flies i believe) or an inactivation of half of the X in females, which is what humans do through the formation of Barr bodies.

 

[phospho]

X is one of the rare exceptions, by virtue of the fact that there necessarily has to be an asymmetry between males and females. girls have 2, dudes have 1. as you know from karyotype pictures, the Y chromosome is tiny - it essentially just codes for dude parts. since X actually codes for somatic genes unlike Y, we need to make up this difference somehow, or women and men would be much different - either a doubling of expression of X in males (a strategy in some flies i believe) or an inactivation of half of the X in females, which is what humans do through the formation of Barr bodies.

makes perfect sense. My last question is about the final part of your last sentence:

"inactivation of half of the X in females, which is what humans do through the formation of Barr bodies"

I thought that barr bodies were something totally different from X-inactivation. X-inactivation is part of the Lyon's hypothesis where as you mentioned, we need asymmetry between males and females, so inactivation of X occurs in females, but nothing happens in males. On the other hand, Barr bodies occurs in both females and males. In the female, it's 3 Barr bodies total, to increase nucleocytoplasmic ratio in that one remaining oocyte. In males, there are 2 Barr bodies total, since out of the 4 spermatids, we need 2: one X and one Y.

Is my reasoning correct, or am I really confused?

thank you for your help! 😍

 

[RogueUnicorn]

makes perfect sense. My last question is about the final part of your last sentence:

"inactivation of half of the X in females, which is what humans do through the formation of Barr bodies"

I thought that barr bodies were something totally different from X-inactivation. X-inactivation is part of the Lyon's hypothesis where as you mentioned, we need asymmetry between males and females, so inactivation of X occurs in females, but nothing happens in males. On the other hand, Barr bodies occurs in both females and males. In the female, it's 3 Barr bodies total, to increase nucleocytoplasmic ratio in that one remaining oocyte. In males, there are 2 Barr bodies total, since out of the 4 spermatids, we need 2: one X and one Y.

Is my reasoning correct, or am I really confused?

thank you for your help! 😍

Hmm? Barr bodies refer to the inactivated X chromosomes (by heterochromatinization of the entire chromosome) in female cells. I think what you may be calling the 3 Barr bodies in oogenesis are polar bodies? However, male gametogenesis produces no polar bodies.

 

[phospho]

Hmm? Barr bodies refer to the inactivated X chromosomes (by heterochromatinization of the entire chromosome) in female cells. I think what you may be calling the 3 Barr bodies in oogenesis are polar bodies? However, male gametogenesis produces no polar bodies.

oops... i feel stupid... you're totally right... thanks again for your help! 🙂

 

[MCAT guy]

looks like you guys got it. Barr bodies aren't in males.

The whole Lyon's thing is a theory talking about Barr bodies.

each of the male gametes has a Y. I'm pretty sure there are no X's in those spermatids.

 

[RogueUnicorn]

oops... i feel stupid... you're totally right... thanks again for your help! 🙂

any time

 

[phospho]

I'm pretty sure there are no X's in those spermatids.

is that always the case? should we assume each germ cell gives 4 Y's? If there was an X or two by accident, would that X be considered a polar body (since it isn't really good for anything)?

 

[MCAT guy]

is that always the case? should we assume each germ cell gives 4 Y's? If there was an X or two by accident, would that X be considered a polar body (since it isn't really good for anything)?

lol, sorry I was just replying off the cuff. The part I bolded is wrong, I said the wrong thing above. There will be two X's and two Y's (which is exactly a duplicate of the male X,Y)

So yeah, there are 4 spermatids should have 2 X's and 2 Y's, which would give you a 50/50 shot at male or female. So the 4 spermtaids go X,X,Y,Y.

The polar bodies only refer to the actual female gametes that don't make it (the 3 that are set to the side to save cytoplasm).

The mature ovum has an X, and I am pretty such each of the polar bodies have X,X,X. They are all just discarded.

Polar bodies are not X chromosomes or any chromosomes. They are the bad female gametes. Remember the inactivated X is only the barr bodies. Which is just saying that we don't need two X's to be actively transcribing genes, only 1 is necessary in each human cell.

 

[phospho]

Polar bodies are not X chromosomes or any chromosomes. They are the bad female gametes. Remember the inactivated X is only the barr bodies. Which is just saying that we don't need two X's to be actively transcribing genes, only 1 is necessary in each human cell.

I keep referring to my notes... now I'm starting to think i'm better off throwing them away... mixing too many things together. I appreciate the heads up.

Thank you!!!! 🙂