h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

On an editorial note, you could do with filling out an intro and grouping bits together to form loosely themed paragraphs of five or six lines length. The content's coming on nicely, though - good work!

Whoami?

A Big SORRY here, I have tried to find enough information for a section on the genetic compounent but I haven't been able to. (due to lack of access to the medschool library- the joys of being a year out because of health reasons student) So I will be unable at the moment to contribute anything.

that's ok, i'll see what I can dig up from the books here. Thanks for trying

SallyM

thanks for trying anyway/

I'm off for my easter hols so I'll update as soon as possible, but it'll probably be about 3 weeks so don't get worried if you've not heard from me

SallyM

Hi SallyM

This seems an interesting entry, although I think there is scope for expanding & explaining some parts a little more. These are a few suggestions, but you are welcome to ignore any that you don't like.

I wonder if "The iris is made up of 4 layers; the anterior border layer, the stroma and a double layer of endothelium" could be edited to indicate that the anterior border was the front layer, facing the outside world and the endothelium was on the inside.

After your paragraph on brown eyes, I would add a little more about melanin, since this is clearly one of the key features. You could try something like: "Melanin is the principle pigment in hair and skin and differing levels of melanin account for the differences in skin and hair colour between races and individuals. People with dark skin and hair have a generally higher level of melanin than pale, blond people. As a result, people with darker skin and/or hair are more likely to have brown eyes".

As to the genetics, you could include a v. basic genetics explanation, without having to go into any great detail. You could try something along the lines of:

"like many human characteristics, eye colour is determined at least partially by genetics. That is, if all your relatives have brown eyes you probably will too!

As with most genes, people have two copies of the gene controlling eye-colour (one from each parent). You only need one copy turned 'on' to make enough melanin to have brown eyes. Obviously the same applies to your parents, so a brown eyed parent could have one or two 'on' copies, but a blue eyed one must have two 'off' genes.

So, if your father has brown eyes, he might have both genes turned 'on', or just one. If he gives you an 'on' copy then you will have brown eyes, whatever colour eyes your mother has. The same applies to your mother. If she gives you an 'on' copy then your eyes will be brown. If you get an 'on' copy from each parent then your children's eyes will be brown too. You can still get blue-eyes from two brown-eyed parents, but only if they both have one 'on' and one 'off' copy and they both give their 'off' copy to you. If this were the case, you should have 3 brown-eyed siblings for every blue-eyed one (although you would need a lot a brothers & sisters to test this).

There are other genes that alter things too, so just because you have brown eyes and two blue-eyed parents does't necessarily mean you should be calling the mailman 'daddy'."

I can't find a dedicated entry on Mendelian genetics but I'm sure one could be written.

By the way, I once met a guy with two different colour eyes (both worked I hope - he was a coach driver). Do you know if this happens often?

Anyway, take or leave any of the above as you wish and let me know if you reckon I can help at all.



Ugi

Heterochromia (or heterochromia iridium) is the term used when the eyes are different colours. Apparently it's usually natural, and is evident from birth, but sometimes it can result from eye disease, so it's worth checking out.
The term is also used where different parts of a single iris vary in colour. (Specifically, heterochromia iridis is the precise term for that.)

See
http://www.optometry.co.uk/articles/19990129/swann.pdf
for information on the disease aspects of heterochromia


I once saw a cat with two completely different coloured eyes, both of which were large and strongly coloured, and it did look extremely strange.

Cheers Potholer!

I had heard that it happened in cats also but haven't ever seen one that I remember.

You looked at this guy & thought he looked funny for some reason & then only later realised it was because his eyes didn't match! I'm sure he would have had it checked out because it was noticable enough.



Ugi

David Bowie, the pop/rock star, has odd eyes.

There are lots of odd things about David Bowie !

Something I've been wondering recently is about whether green eye colour is dominant or recessive. It's well known that brown is dominant and blue recessive, but does anyone know about this one?

Jen

Unlike blue eyes, there seems to be a gradation between green and brown eyes - there are some eyes that are definitely green, some that are definitely pale brown, and some darker 'brown' eyes that could be either dark brown or dark green. (Dark brown is a bit of a vague colour)

Even in the case of blue eyes, there are some that are basically grey-blue, and some that are seriously blue. Some grey eyes seem to have a minimal (even undiscernable) blue component.

I'd guess that there are several genetic controls - possibly one recessive one (or more) that mask out the blueness or greyness, and one or more others that control shading.
The full picture could be very complex - I rememebr reading about the coat coloration genes for horses, and there was a whole cascade of recessive/dominant genes that affected the overall colours and patterns.

Perhaps footnote 'irides' to say that this is the correct plural of iris?

> "The iris is made up of 4 layers; the anterior border layer, the stroma and a double layer of endothelium"

Which is on the inside, which is on the outside?

Are coloured contact lenses completely off-topic here?

Just to let you know that this entry has been scouted - can you let me know when all the updates have been included so I can process it?

Many thanks

Ashley

Hi SallyM

An intersting read, thanks

Just one area that confuses me... which is a miracle!

From what little I know about the eye, I gather that when the pupil is constricted less light will be *reflected* (back out) through the pupil than when dilated but... the way I read your piece, when the sun/light level is bright, *less* light enters the eye.

Am I wrong in thinking the opposite i.e that when the pupil is constricted due to bright light levels, *more* light enters the eye and this is why the world looks bright?

six7s

ps
<>
should this be *e.g* instead of *i.e* ??

(Assuming the pupil isn't already contracted, and that the higher light level is sufficient to cause contraction) When light levels increase from amount A to amount B, the pupil is constricted, and less light enters than if it hadn't constricted, but generally more light will still enter at level B than level A. The contraction *attempts* to counteract the increase, but will not compensate completely.

Given that min/max pupil sizes are something like 2mm/8mm, the most difference the pupil can make is in the order of a 16-fold reduction in light levels. Bright light can be millions of times brighter than low light, so even allowing for transit in two directions through the pupil, in absolute terms more light will be reflected from the inside of a brightly lit eye with a constricted pupil than from a dimly lit eye with a dilated one.

In fact, by far the greatest ammount of adaptation to varying light levels is accomplished by the reversible bleaching of photosensitive pigments in the retina - in bright light, the vast majority of the pigment is in the bleached state, and does not respond to light, and as light levels drop, an increasing amount of the pigment recovers to the sensitive unbleached state, and so the sensitivity of the retina rises.

Even in the virtual absence of light, the recovery process can take a long time to run to completion, which accounts for the ~10 minute interval when moving from daylight into darkness before full night vision is achieved. (An experience well known to cavers)

Cheers Potholer!

Thanks for the prompt reply

six7's

Well what do you think? Typos, more detail etc. I was going to include the bit about pupil dilation but it seemed to go off the subject.

Sitting here with bated breath

SallyM (almost spot on with the 3 weeks, have been to Isle of Wight and Isle of Man)

Given what the article is about, (eye colour, rather than the entire human eye) I think sticking to what can be seen from the outside is probably a good idea.
I hope I haven't implied anything else by what I wrote. I do tend to get a bit carried away with science sometimes, and hate giving half an explanation when two explanations will do. I also find it hard to resist a question, even when people haven't quite got round to asking it yet.

That's quite alright, kinda needed a bit of help and you've been a

SallyM