sort of genetics question - given dolly the sheep, why am I not identical to my brother?

[lummox]

I meant to post this on the silly questions thread but now can't find it.

Dolly the sheep (as I understand it) was created from one cell, and any cell could have made a whole Dolly the sheep.

I was made from one cell of my mother and one of my father. My brother (so I have always been told) was also made of one cell from each of those people.

So I can't work out why we are not identical. Does anyone know?

OK, there are real genetics experts on MN who will do a better job, But I will fill in for a bit

Dolly the sheep was made when all the genetic information in one sheep was cloned.....made to grow into a whole new sheep. While new, all the genetic information was identical to the original sheep.

When we make out eggs or sperm, half of our information goes into them. Our normal cells have 23 pais of chromosomes, each sperm or egg cell has one of each pair.....

now here in the snazzy bit, it is random which of each of the pair of chromosomes end up in the sperm or egg cell. Each of the pairs of chromosomes carries slightly different versions of the enformation. So one of a pair migh carry the infomation to make blonde hair, and the other of the pair migh say make brown hair. It is random chance which of each pair ends up in the sex cell. Bits of information also get swapped between pairs of chromosomes, called crosiing over and this also adds to the mix

So all of your eggs carries subtley different infomation, and so will your parners sperm. So whan the join to gether they may similar, but not identical children

apparently it is possible to have siblings born seperately who are genetically identical, the same way identical twins are, but it has never happened in recorded history as the odds are god knows how mant millions to one

hey thanks. I'm still not sure I understand why or how eggs and sperm can be random creations though, if a person only has one lot of genetic information (sorry, slightly lame finish there).

x-posted waterfalls. does that mean that there is a chance that two of my eggs/partners' sperm are identical?

Just as MB says.

Dolly the sheep was special as she was made from one cell of another sheep - she had only the one 'parent' so all of her chromosomes were the same as that parent and she was genetically identical to the parent. therefore she was a clone.

You have got 50% of your mother's genetic information and 50% of your father's genetic information, so has your brother ..but..it's not the same 50%.

As MB said, there is 'random segregation' of the chromosomes when the egg/sperm are made so although the chromosomes divide equally so you get one of each chromosome pair, there are lots of different variations of which chromosome combination you get.

On top of this, the chromosomes get together before they divide into new cells and swap bits of information within the pairs so even if you inherit the same chromosome as your brother the chances are that it is altered slightly compared to your parent, and his is altered slightly compared to yours and your parents.

OK, Imagine that you have 23 pair of coins, one head and one tail for each.

When sperm and egg cells are made, one of each of the pairs goes into the new cell. Which one, head or tail is random. In actual fact what happens is that all the choromosomes line up in thier pairs in the middle of the cell, randomly, but in their pairs. They attach to microsopic threads that pull one of each of the pairs to opposite ends of the cell. the cell then splits in two.

So you could get

all heads on one side and all tails on the other

or 22 heads on one side with one tail on one side and 22 tails with one head on the other

oe 21 heas and two tails on one side and 21 tails and two heads on the other

and so on and so on

All down to random chance

All eggs and sperm have the same cells ( i think), it just depends what is selected at conception.

look at this animation
\link{http://www.lewport.wnyric.org/jwanamaker/animations/meiosis.html\here}

One cell of 46 chromosomes makes 4 egg or sperm cells , each with 23 chromosomes.

When egg and sperm join, you ger back to 23 pairs.....46 chromosomes

That animation is brilliant. Thank you.

Also bluebear, thank you for explaining why dolly the sheep was different.

Knew that someone out there would know the answer.

Trying to think of a simpler way to explain it.

If you have 2 red socks (with left and right written on them) and 2 blue socks (with left and right written on them) and you have to give one red and one blue to your child you have a choice of;

red left and blue left
red left and blue right
red right and blue left
red right and blue right

so each combination gives 50% of the socks (2 out of the original 4) but if you gave combination 1 to child 1, combination 4 to child 2 then although both would have 2socks, 1 red 1 blue, they would have completely different socks.

Imagine how many combinations there are when there are 23 pairs instead of 2.

Think what I'm trying to say is that although chromosomes come in pairs, the information on each one of the pairs is not identical, so inheriting one of the pair is not the same as inheriting the other one.

Coins much better than socks MB! Well done,

Lol, just read thread title again................your not identical to your brother, because he is male and you are female, lol

At least 3 people were adament when I was expecting identical twins, that you can have identical boy and girl one person even claimed to know identical twin boy and girl

So that would mean that it was possible to have genetically identical siblings born at different times?

can you spot the scince teacher who has been doing this as revision with her year 11s? Over, and over again?????

waterfalls - i knew i wasn't really identical (as well as being a different gender he is much fonder of coffee than I am).

ooh, could I ask another similar question (although this might stary too far into pop science).

how far does looking like someone mean that you are likely to be genetically similar? so if I have two full siblings, and I look like one but not the other, am I likely to have a greater genetic similarity to that one?

I know, it would just be funny if a serious question

MB - I only have to teach it to graduates who should really understand it already!

Lummox - it is extremely enormously unlikely (as Lola would say) that genetically identical siblings would occur without them being identical twins.....but it is theoretically possible, as waterfalls said below.

sorry waterfalls, on re-reading my reply it seems a little humourless - was meant to be funny!

Lummox, re the similarity bit, my dh is one of 4 brothers. He has been tissue typed with them, and he is a gentic match with the one who he most closly resembles....but this is IIRC a genetic fluke. What we look like is controled bt so many genes, the varientions are almost endless.....what we look like is a slo shaped by our enviroment.

even geneticaly identical twins, who have the same chromosomes and genes have different fingerprints, as each had a sligly different position in the womb.....and that makes a differenece in the finger prints!

Isn't biology amazing!

why are identical twins not entirely identical?

and why do some voices within the same family sound the same? People often mistake dd for me for instance on teh telephone

Jampots, because what we look like is shaped not only by out genes but the enviroment. Twins will have different environmnts and this will affect what they look like. They start off very, very similar and as they get older become more dissimilar