## If two identical twins produced an offspring (gross), would the offspring be some kind of genetic clone?

If two identical twins produced an offspring (gross), would the offspring be some kind of genetic clone?

This wouldn’t be possible since identical twins would be the same gender.

If this was possible (other people have mentioned the same-sex thing) then the child would not be a clone of the parent(s).

Any genes where the parent was homozygous would still be homozygous. However, only 50% of the genes where there is heterozygosity would still be heterozygous. The others would become homozygous.

It’s this increase in homozygosity which causes the negative effects of inbreeding.

Actually, it is possible for identical twins (twins born through a process where a single fertilized egg splits into two) to be born with different genders in the presence of genetic defects. If the gene determining gender is xxy, when the egg splits it is possible to have one xx and one xy egg after the fact.

This is absurdly rare, but it is possible.

No. Imagine each identical parent has a dominant trait, but they also carry the recessive gene for that trait, Dd. D being the dominant gene, and d being recessive. A child gets only one copy of each gene from each parent, so it could be that said child would get dd, and display the recessive trait.

## If temperature is a metric for the average kinetic energy of particles, is there also a metric for the standard deviation of the kinetic energy of particles?

If temperature is a metric for the average kinetic energy of particles, is there also a metric for the standard deviation of the kinetic energy of particles?

What factors determine this standard deviation?

Temperature is not really a direct measure for the average kinetic energy. It's more that both the average and standard deviation are related to the temperature. For an ideal gas, this relationship is given by the Maxwell-Boltzmann distribution. As the temperature increases, the average kinetic energy increases, and the width of the distribution increases too.

The definition of the temperature is

1/T = (dS/dE), where the derivative is a partial derivative, and the other extensive quantities are held fixed. S is the entropy and E is the internal energy.

For a classical ideal gas, the equipartition theorem gives the relation

<K> = 3kT/2, which allows you to relate the temperature to the average kinetic energy of the gas molecules.

Maybe a definition of temperture would be useful here?

A classical ideal gas at equilibrium obeys the Maxwell-Boltzmann distribution:

f(E) = 2(kT)-3/2*sqrt[E/π]*exp[-E/kT].

The mean of this distribution is

μ = 3kT/2.

The variance is defined by the average value of (E - μ)2 which is just

[; \int_{0}^{\infty} f(E) (E-\mu)^{2} dE = \frac{2}{\sqrt{\pi}}(kT)^{-3/2}\int_{0}^{\infty} \Big(E - \frac{3kT}{2}\Big)^{2} \sqrt{E} \exp \Big(-E/kT\Big) ;].

If I calculated it correctly, this whole thing evaluates to 3(kT)2/2. That's the variance of the kinetic energy, so the standard deviation is sqrt(3/2)kT.

## How "green" is the life cycle of a solar panel end-to-end compared to traditional energy sources?

How "green" is the life cycle of a solar panel end-to-end compared to traditional energy sources?

The 2014 IPCC climate change assessment report actually included a life cycle assessment of CO2 equivalent emissions from different electricity sources.

Here is a nice table on Wikipedia, and here is the full report if you'd like to look into the specifics.

Solar panels are significantly better than traditional energy sources in this regard.

Sorry to piggyback on the top answer, but if anyone's interested in impacts other than carbon footprint, here's an answer I wrote further down:

I wrote a paper that's open access, which you can find here: http://www.sciencedirect.com/science/article/pii/S0306261914008745

The TLDR is that solar is good in terms of climate change but generally less good in terms of other impacts. Overall it's typically not as good as wind or nuclear power. However, bear in mind two things:

Impacts are very dependent on location. E.g. solar installed here in the UK is worse than in Nevada or Spain because the impacts are all up-front but you get much more energy output in sunny countries, therefore better impacts per kWh. Solar technology is moving fast. I actually have some updated figures that I'd love to share with you but they're not published yet, so they're not peer-reviewed. Reductions in impacts in the past few years have been considerable: about 50% reduction between 2005 and 2015. So for instance I now estimate a carbon footprint of about 45 g CO2-eq./kWh for a UK installation or 27 g in Spain. This contrasts with the figure of 89g you'll see in the paper I linked.

Well like many industrial processes there are some fairly nasty chemicals involved, so to an extent we're relying on good regulation. There have been a few cases in China where various toxic spills have occurred from solar plants (e.g. see here).

There are also some issues with specific types of solar power, e.g. CdTe cells or CIGS cells, which contain cadmium. As you probably know, Cd is very toxic. There's currently a lot of work going on to eliminate Cd from all consumer equipment, so this should be solved in time.

In context though, it's important to remember that all of these impacts tend to be minimal compared to the environmental and health implications of coal power. So as long as we're replacing coal, it's still easily a benefit. And, of course, solar isn't much different to many other high tech industries, so it's not like the issues I mentioned above are uniquely problematic.

Finally something I can help out with! Source: I'm a lecturer in the UK (roughly equiv. to assistant professor in the USA) specialising in life cycle assessment, particularly energy sources.

I wrote a paper that's open access, which you can find here: http://www.sciencedirect.com/science/article/pii/S0306261914008745

The TLDR is that solar is good in terms of climate change but generally less good in terms of other impacts. Overall it's typically not as good as wind or nuclear power. However, bear in mind two things:

Impacts are very dependent on location. E.g. solar installed here in the UK is worse than in Nevada or Spain because the impacts are all up-front but you get much more energy output in sunny countries, therefore better impacts per kWh. Solar technology is moving fast. I actually have some updated figures that I'd love to share with you but they're not published yet, so they're not peer-reviewed. Reductions in impacts in the past few years have been considerable: about 50% reduction between 2005 and 2015. So for instance I now estimate a carbon footprint of about 45 g CO2-eq./kWh for a UK installation or 27 g in Spain. This contrasts with the figure of 89g you'll see in the paper I linked.

## What % of my weight am I actually lifting when doing a push-up?

What % of my weight am I actually lifting when doing a push-up?

Your question made me curious and a quick search yielded the study linked below, which looked at exactly this question.1 The researchers found that the answer depends both on the variant of the exercise as well as the stage of the exercise. For example, in a traditional push-up the number is about 69% in the up position (at the top of the movement) and 75% in the down position (bottom of the movement).

It's also worth mentioning that the study also looked at a "modified push-up." This modification is essentially just an lazier easier version of the exercise where the knees stay on the floor. Surprisingly (to me at least), even in this simpler version you still lift quite a bit of your body mass (54% in the up position and 62% in the down position).

edit: I corrected "going up/down" to "up/down position" to reflect the fact the body was kept stationary when the force was recorded in this study.

1 Suprak, et al. The effect of position on the percentage of body mass supported during traditional and modified push-up variants. 2011: 25 (2) pp 497-503 J. Strength Cond. Res. Link

You can also modify pushups in the other direction, making them significantly harder (mostly through increased leverage):

hands together pushups forward lean pushups (putting your center of gravity forward, increasing both leverage on shoulders and total body mass lifted) incline pushups (mentioned by others) handstand pushups planche

Note: at no point do you lift 100% of your own body mass, since your hands and forearms are always at rest on the ground.

To measure yourself: Put a bathroom scale under one of your hands while doing a push up. Double the maximum value the scale lists and divide that by your total weight (and multiple by 100) to calculate the percentage.

Someone doing handstand pushups is probably lifting over 90% body mass though, right?

## AskScience AMA Series: I am Dr. John Jangy and I'm here to talk about Peto's Paradox and why larger mammals don't have higher cancer rates. Ask Me Anything!

AskScience AMA Series: I am Dr. John Jangy and I'm here to talk about Peto's Paradox and why larger mammals don't have higher cancer rates. Ask Me Anything!

Hi Dr. Jangy.

What are the current ideas on why larger mammals don't have higher cancer rates and which idea do you think makes the most sense?

How many bowls of chicken soup can fit into a blue whale?

## Is a single Elephant's skin cell bigger than a human's skin cell?

Is a single Elephant's skin cell bigger than a human's skin cell?

No, while cells can come in various sizes there's something called the Square-Cube law that basically means that the larger an object becomes, its volume grows faster than its surface area. This might sound obvious but because cells have to traffic things into and out of their surfaces in order to process nutrients, the ability to transport becomes a limiting factor as the cells can no longer uptake fuel and get rid of waste fast enough. Bacterial cells are tiny and lack sophisticated transport mechanisms within their cell, they tend to stay small because a lot of their molecular transportation is passive in some way. Eukaryotic cells get around this by having organelles, so they have become more efficient at utilizing resources and as a result, can be bigger. However, eukaryotic cells in general also have an upper limit to size. You might notice that this is the reason why some organelles and cells will become wrinkled heavily (like mitochondria, or even organs like the brain) and that's to increase their surface area to volume ratio.

Elephants get around this limitation by simply just having more cells.

To add to this, you would think that, if any cell has a given probability of becoming cancerous, the larger the animal is, the more likely it would be to get cancer. However, this is empirically false. It's called Peto's Paradox. No one is quite sure why that happens, but some interesting hypotheses I've seen is that whale tumors grow their own tumors and die; and that cancer is an adaptation that selects against energy dissipation, i.e. it's not a disease per se but more like the sickle cell mutation: disadvantageous in general but protective in areas with malaria.

Actually there's some work going on right now into this (I am finishing my PhD in tissue engineering and cancer biology this year) and it's exciting. For examples, elephants have many copies of p53, also known as the guardian of the genome (humans have 1 copy of it). It seems that larger animals may indeed have redundant tumor suppressor genes and more guards against oncogene activation.

Of course, the killer whale example is also possible-that tumors in larger animals just operate differently. It's still very interesting.

To add to this, there's also the interaction between p53 and Mdm2; Mdm2 is a negative regulator of p53, strongly conserved from lampreys to humans.

## Why do clothes feel crunchy when you air dry them, but soft out of the dryer?

Why do clothes feel crunchy when you air dry them, but soft out of the dryer?

Tap water has minerals dispersed in it in solution. When it dries on a line the fibers/threads stiffen because the clothing is not moving much, and the threads retain their shape as the water evaporates and leaves the minerals behind, while in a tumbling dryer the threads are constantly crunched and tugged and squeezed so they cannot stiffen in this way.

i'd say even then tumble drying will be softer as the fabric fibres are massaged while drying, and won't stick together.. kinda like how wet pieces of paper stick together if they dry while touching.. water allows the little hairs on the fibres to tangle and hold like velcro, whereas agitation while drying breaks these holds

So if you were to use distilled water for washing your clothing, air dried clothing would come out the same as dryer-ed clothing?

Y'know that lint in the drier? That's literally the result of your clothes being mechanically worn away by the action of the drier. Add to that the fact that getting cotton hot and very dry causes it to break more easily, then yes air dried clothes will absolutely outlive machine-dried clothes.

## Is there a limit to how many elements there can be?

Is there a limit to how many elements there can be?

Yes, probably. But we don't know what it is, and it's likely higher than what we're currently capable of producing experimentally.

The IUPAC has strict criteria for what can be considered an "element". One of this criteria is that it has at least one isotope with a lifetime at least on the order of 10-14 seconds.

As you go up in Z around the superheavy elements, they become very unstable to alpha decay and spontaneous fission. If you extrapolate the lifetimes to heavier species based on trends in nuclei that we know of, eventually you'll probably reach a point where no heavier species has a lifetime which meets that criterion in the definition of an element.

That's approximately the timescale of the "motion" of electrons within an atom. In other words, it's the time needed for a nucleus to grab onto electrons and form a chemically stable atom.

If it doesn't live long enough to do that, the chemists don't want to call it an element.

Why 10-14? Is there something fundamental about that number or is it just a line in the sand?

So is it a little like "the planets have to make it around the sun a couple times or else its not a 'solar system' just a star with some asteroids whizzing by today" but on a subatomic level, yeah? Is that a not-completely-wrong analogy?

## How much does drinking a cold drink really affect your body temperature?

How much does drinking a cold drink really affect your body temperature?

I'm an anesthesiologist. We monitor body temperature during surgery because anesthesia inhibits your ability to autoregulate temperature. Essentially you are turned into a poikilotherm like a snake, and lose heat to the cold operating room. An inability to contract your muscles prevents you from generating heat. We have a rule of thumb that 1 liter of room temperature intravenous fluids will reduce a patient's body temperature by 0.25 degrees Celsius. We used forced air warming blankets and heated IV fluids to maintain a normal body temperature, which helps the body to metabolize medications predictably and the blood to clot properly.

After reading comments I want to add that the reason I brought up anesthesia here is that only when you remove the body's ability to generate heat can you actually measure a reduction in temperature, unless you infuse the fluid very quickly. When we drink cold fluids, the body generates heat to correct the drop in temperature before an appreciable difference can be measured.

Furthermore, there are some interesting studies out there on this. Many involve rapidly administering cold IV fluids in attempt to show that hypothermia is protective against neurologic injury in situations such as cardiac arrest.

Here is one study:

Ann Emerg Med. 2008 Feb;51(2):153-9. Epub 2007 Nov 28.

They infuse cold and room temperature fluids rapidly in non anesthetized patients and measure a temperature change before compensatory mechanisms (shivering) can restore the body to normal temp. This is better than my rule of thumb as it uses weight-based dosing for IV fluids. Interesting, 30ml/kg of room temp fluid reduced the body core temp by 0.5 Celsius degrees. That would be 3 liters of fluid for a 100kg (220lb) person. Cold fluid reduced the body temp by a full degree Celsius.

That's really interesting, thanks

We can do a very(!) rough back of the envelope calculation.

Assume a 100kg person (I like round numbers). Assume that they're all water, so we have 100 litres. Assume that they're at body temperature, so about 310 Kelvin.

Now you drink 0.2 liters of ice cold water, 273 Kelvin.

Since both are water, they'll have the same heat capacity and the end temperature will be just a simple weighted average:

T = (100 * 310 + 0.2 * 273) / (100 + 0.2) = 309.93

so it's almost negligible, like a 0.07 degree drop.

If you wanted to be more accurate, you could use the average specific heat capacity of the human body. I can find it via google, but that would take the fun out of computing it. You'd use a weighted average of the capacity of water (60% of human body is water) and of things like proteins, fat, bones.

It wouldn't drastically alter the equation though, the fact that the drop in temperature would be small will remain.

Like, let's use the factor 0.5:

(50 * 310 + 0.2 * 273) / (50 + 0.2) = 309.85, so now we're looking at a .15 degree drop. Still negligibly small.

Long story in short: There are some heat receptors in stomach helping your body determine sweating and drinking hot beverages may freshen you since you sweat more (and if the place is windy so that your sweat would vaporize, unless you just feel hotter) and drinking cold beverages lessen your sweating after an instant cooling so it depends on the environment. If the place is chilly/windy like in front of a fan, hot drinks better. But most of the time cool beverages are the best.

## Does the Doppler effect have any noticeable consequences on wireless Internet connection?

Does the Doppler effect have any noticeable consequences on wireless Internet connection?

Let's say I'm downloading a file and run towards my router at significant speed, will that make the file download faster, or cause errors? Does it matter whether the signal is AM or FM?

Yes it can actually have a pretty significant effect on communications. In particular, if S(f) is the spectrum of your original signal, S(af) will be the spectrum afterwards (PDF). Here a is relatively close1 to 1, basically 1 - v/c where c is the speed of light and v the velocity of the object. At most the shift will be a few Hz.

The shift in Hz is not a problem, in fact phase lock loop are already usually applied so that the receiver can reap the gains of using coherent demodulators (PDF) see here (PDF). Phase lock loops help to account for artifacts such as signal drift, and so on. The point being this is not where the problem comes in.

The big problem is what occurs with multi-path communications. If your signal has multiple reflections (like, if you are in a room of some sort instead of in an open field). When moving now, there will be a different velocity shift depending on the angle of arrival of the wave. And because of this the signal, being the sum of the different waves, can experience a wide arrange of channels. Some of these channels will begin to have different interference patterns. This is problematic though as the capacity (maximum amount of bits/second that can be sent over it) basically defaults to the minimum rate achievable over all possible channels simultaneously (see Theorem 1 (PDF)). Thus if one of those bands you pass over has a really bad channel, there goes your ability to connect in general. While the variation due only to doppler shift will be small (about 80 Hz for 5 mph over 5GHz), it is just on the edge of what is considered to not be completely trivial.2

Anytime you transmit a code with a higher rate than the capacity of the channel, an error will occur. As a result a moving source does increase the probability of packet error, with the probability increase depending on the speed and strength of the reflections.

The same principal applies to AM and FM radio. Although both of which are horribly inefficient in terms of transferring information, so it is unlikely that it would have a noticeable impact. To put it in perspective, a WiFi router is trying to transmit at the very edge of comprehension, while AM and FM require the signal to be easily understandable. Adding a little bit more noise to the WiFi signal will move it past the point of comprehension, while for AM and FM the added noise will still result in a large enough SNR for it to not make a difference.

Edit--

1 - correction due to /u/devman0

2 - Added later, as was pointed out by /u/dirtyuncleron69 and /u/dayzstu. Crossed out what is not directly attributable to the doppler shift.

Also, while on the edit, for a much more in depth discussion about how we model movement and the impact this has on communication, please see Wireless Communications by Andrea Goldsmith. She is the most prominent authority on the subject. Chapter 3 in particular talks about this exact scenario in realistic environments, and chapter 5 the impact on capacity.

so, this is shift from reflections being out of phase with the original signal, due to a longer path, and those reflections changing with time due to the source moving.

If there were zero reflections, say I'm just in an open field with a router and my phone, would this effect still occur?

Your answer is wrong and you know it. It seems like you forgot what the question was as soon as you started typing.

Does the Doppler effect have any noticeable consequences on wireless Internet connection?

Yes it can actually have a pretty significant effect on communications.

... The shift in Hz is not a problem

So the doppler effect is not a problem.

The big problem is what occurs with multi-path communications. If your signal has multiple reflections (like, if you are in a room of some sort instead of in an open field). When moving now, there will be a different velocity shift depending on the angle of arrival of the wave.

This is not the doppler effect.

You answered "Yes it can actually" but your reasoning only supports "No, but other factors can..."

This is not the doppler effect.

You know, you are right. That is not the doppler effect. Yeah my bad. But I am not saying other factors, the doppler effect in and of itself still reduces the capacity of a channel in a multi-path environment in a way that does not occur with LOS.

The doppler effect in multi-path environments can be modeled as a random time-varying channel. This is primarily because it would be very hard to determine the velocities relative to every individual reflection arriving at the receiver. In essence then, the signal will be as if there was a random perturbation in the frequency. Albeit small, this random perturbation is then a source of noise which generally decrease the capacity of the channel. Thus the combination of doppler and multi-path do in fact work together to degrade the channel. With LOS this is not the case as any constant movement would only invoke a constant doppler shift. Furthermore, the capacity of any arbitrarily varying channel is generally less than the minimum of the capacities of the possible respective channels.