Recent comments in /f/askscience

fuscator t1_j4kdujj wrote

Reply to My 4y/o told me her dream about losing teeth without ever knowing it's a common dream. Are there any other examples of cross-cultural phenomena that centers on a specific narrative? by [deleted]

Common dreams for me, anyone else?

Realising you have an exam today and you haven't studied.

Running, but not able to run properly, like you have lost motor control. Mostly running some sort of race, sometimes away from something.

Realising you've gone bald (I have relatively full hair, male)

Realising you don't have trousers or underwear on, in a crowd (not had this for many years, it was when I was younger).

Hmm, it seems maybe I have insecurity issues 🤣

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Ordoshsen t1_j4kck6e wrote

Reply to comment by IAmAPhysicsGuy in How do we know that we are in a certain place inside our galaxy? and how do we know how big it is just by looking at the cross section we are in? by friday_panda

> Your brain can calculate distance because of the parallax that you get from having two eyes.

This only works for stars that are very close to us, not for most of the stars we see.

> Or you can see that people look smaller when they are further away (assuming you are familiar with the average size of a person).

If you use average size you're going to have a large error. You need to know how large the star actually is to determine how far it is. You can't just assume all stars are the same on average because we know that's not the case. Also I assume you're actually talking about apparent and absolute magnitude here.

I'll just add that this can be part of the puzzle when using dynamic paralax, but it doesn't work as you have worded it.

> Imagine these same principles applying to different positions in orbit around the Sun throughout the year

Yes, parallax works like that. But you will see no change in brightness of a star on our orbit.

> type 1A supernovae, and red / blue shift of light

Both of these are used for measuring distances to other galaxies. You can tell how far a Ia supernova (standard candle) has occured. But if you happen to see one from our own galaxy, you just know there was one binary star there but generally we just look at a bunch of galaxies and hope to see one there. But you probably didn't even know there was a star there before so you can't use that to measure stars you actually knew about.

red blue shift can tell you distance only of galaxies outside our neighborhood. You can't use it for measuring distance of stars that are next to us.

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e_j_white t1_j4kcf6n wrote

Reply to How do we know that we are in a certain place inside our galaxy? and how do we know how big it is just by looking at the cross section we are in? by friday_panda

Great answers here, just want to add that we don't "know" for sure. We're still trying to measure the size of Milky Way every year.

We thought it was 100K ly across, but some researchers in 2015 claimed it's 150K ly.

Just recently, a new publication is now saying it could be 200K ly across!

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atomfullerene t1_j4k7a8g wrote

Reply to Can a recessive gene become a dominant gene? If so, how long would it take? by Pretend-Recover-4418

So first of all, it's not genes which are dominant or recessive, it's alleles. Alleles are variants of genes. For example, there's a blood type gene with alleles A, B, and O. You generally have two alleles of each gene, since you have two copies of (most) genes.

Alleles are dominant or recessive as a side effect of how they work. For example, consider a gene that's involved in melanin production. It's got an allele that makes melanin normally. And it's got an allele with a mutation that makes it not work. A cell will activate this gene if it senses the cell doesn't have enough melanin.

If you have two copies of the working allele, you get melanin. If you have one copy of the working allele and one of the busted allele, you get melanin. If you have two copies of the busted allele, you get no melanin. So the working allele is dominant.

Now, to get back to your question, can this change? Well...no. Because if an allele changes, it becomes a different allele. An allele is a specific version of a gene. If you mutate it in some way, you make a new version of that gene, a different allele.

Dominance and recessiveness aren't even absolute things, they depend on which other allele you are comparing to. For example, go back to A B O blood types. A is dominant over O, but it's not dominant over B. If you have A and O alleles, you just have type A blood. If you have A and B alleles, you have AB type blood. So is A dominant, or not?

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