Why don't we make clones during sexual reproduction?

[simonecuttlefish]

Sorry for being so incredibly stupid but here goes. I thought I was "a bit" across the sexual reproduction thing. But I was thinking about it and suddenly a massive void appeared and it all fell in. Here is my problem.

Woman:
Eggs are present from birth and are not "produced" fresh (like sperm are). The eggs should be either her fathers genetic compliment or her mother genetic compliment, from the split chromosome pairs that make her up. I was assuming her eggs are made from either 1/2 of the DNA strands that bonded when she was conceived.

Man:
1 cell divides and replicates, then divides again but does not replicate, so it becomes 4 sperm cells. Those sperm cells would either be a set of his mothers, or a set of his fathers chromosome 1/2's.

SO - if this couple have 2 children of the same sex, shouldn't there be a 1/4 chance they would be genetically identical, even though they were conceived at different times?

SEE! I said I was stupid.

 

[AndromedasWake]

Well firstly your parents don't look the same, and since you receive a mix of two parent genomes, not all of the genes can be expressed. Dominant genes are expressed over recessive ones, so in general you inherit different traits from each parent. Most children are strikingly similar in appearance to one or both of their parents.

You aren't a clone because you aren't genetically identical. A real number of genetic differences exist between you and each of your parents, and they exist due to mutations. The only mutations which are important are the ones you inherit; the ones that occur in the DNA in each sex cell.

The number of mutations compared to the overall size of the genome is tiny, so only tiny changes occur with each generation (in general, although of course a very critical mutation might occur in a single generation this is improbable.)

These mutations over many generations will result in children who look almost nothing like their great great great great (etc.) grandparents. Go back about 350,000 generations and your organism looks so unlike a modern human, that it would actually go on to become the ancestor of chimpanzees as well.

Chimpanzees only have fairly broad anatomical similarities to humans, but you could consider them and humans to be "children" of a single parent (their common ancestor.)

You'd never notice it in real time, but those gradual changes which prevent us from being clones of our parents are what lead to such diversification over about 5 million years.

tl;dr If everyone was a clone of their parents, there wouldn't be any evolution!

EDIT: I may not have made this clear, but the mutations occur in the actual DNA itself. Even if the DNA in the egg is made at birth, it will still change over time. It may be subject to spontaneous mutations or influenced by a mutagen. In theory it's possible that a cosmic ray can become a mutagen, meaning you could have been altered by some distant object in the galaxy before you were born!

 

[Deleted member 619]

^^ I think you misread, mate. I understood his question to be 'why are the offspring of a single pair of parents not clones. I read it the same way you did at first, and then realised I must have read it wrong.

If I have read it right, here's my response:

No, the way that DNA recombines, and the sheer vast number of ways it can recombine, almost preclude that. It doesn't preclude it completely, as it is entirely possible for the DNA to recombine in precisely the same manner twice, but the odds against it are astronomical.

So, while not impossible, I think, incredibly unlikely. Bear in mind the sheer number of links in the ladder. If even one of them is different in the latter offspring, you don't have a clone, and the reality is that a good deal more than one is going to be different in pretty much every case.

I know a genetecist, and I'll ask him if he has any idea what the odds are, but they're going to somewhere on the scale of the odds of a tennis ball passing through a wall, I should think, which would mean that such an occurence would be unlikely to happen in the life expectancy of the cosmos.

 

[Aught3]

Independent assortment.

While it is possible that a gamete will contain only paternal or maternal chromosomes, much more likely it will contain a mixture. There are 23 pairs of chromosomes, one from mum and one from dad. When these chromosomes segregate during meiosis it is random so in each gamete there is a 50/50 chance that chromosome 1 will be paternal and an independent 50/50 chance chromosome 2 will be paternal and so on for the next 21 chromosomes. This means there are a huge number of different gametes that could be created and the likelihood of two identical siblings is vanishing small. Add in the mutations and recombination that occur and it will basically never happen.

Sorry if this is a bit confusing, it's late. Google 'independent assortment', I think that is the answer you are looking for.

 

[AndromedasWake]

Reading back, I'm sure I did take this question the wrong way. Sorry OP! :facepalm:

 

[borrofburi]

Independent assortment.

While it is possible that a gamete will contain only paternal or maternal chromosomes, much more likely it will contain a mixture. There are 23 pairs of chromosomes, one from mum and one from dad. When these chromosomes segregate during meiosis it is random so in each gamete there is a 50/50 chance that chromosome 1 will be paternal and an independent 50/50 chance chromosome 2 will be paternal and so on for the next 21 chromosomes. This means there are a huge number of different gametes that could be created and the likelihood of two identical siblings is vanishing small. Add in the mutations and recombination that occur and it will basically never happen.

Sorry if this is a bit confusing, it's late. Google 'independent assortment', I think that is the answer you are looking for.

This, only there's an added complexity of chromosomal crosssover, in which a gamete often does not get a maternal chromosome, or a paternal chromosome, but a mixture of both. And on top of that each child will have about 150 mutations, which are extremely likely to be in entirely different places (although it is also likely that many of them will be in the junk DNA).

 

[biology4life]

As others have pointed out you have missed two key things about meiosis which is the name given to the process by which gametes (sex cells like sperm and eggs in us humans) are produced.

Meiosis consists of two cell divisions meiosis I and meiosis II. I'll follow the formation of sperm as egg formation has some interesting, but irrelevant to your question, complications.

Before the original cell starts to divide the DNA is replicated but the two copies stay attached together, each copy is called a (sister) chromatid, interestingly at this point there are effectively 4 copies of the genes. During the first part of meiosis I, called prophase 1 the DNA supercoils to form the distinctive 'x' shape where each side of the 'x' is an identical chromatid.
At this point the matching pairs of chromosomes, one of maternal origin the other of paternal origin, come together to form what is called a bivalent. The non-sister chromatids from the two chromosomes attach to each other at points called chiasmata. At some of these points the chromosomes break and recombine with the other chromosome. This means that sections of maternal and paternal origin chromosomes get swapped over.
Later during anaphase 1 the the bivalents are separated, but this is independent segregation it is not a case of all the maternal chromosomes go one way and all the paternal the other, also remember that the chromosomes may not really be maternal and paternal any more as sections may have been exchanged.
At the end of meiosis I there are two daughter cells each containing a single set of chromosomes, each chromosome does however consist of 2 chromatids so there are effectively 2 copies of all the genes.
Meiosis II is in many respects very similar to the normal cell division called mitosis. During meiosis II the sister chromatids will be separated and once again it is a process of independent segregation.
This results in 4 sperm cells each with genes that have been randomly shuffled during meiosis I and randomly dealt in meiosis I and in meiosis II and a single set of genes in each cell.

I hope this explanation helps.

Edit: Re reading your OP you seem to be under the misunderstanding that each 'side' of the DNA double helix of a chromosome comes from each parent, it is not like that. The two sides are complimentary and they only separate to replicate or for the DNA to be 'read'. It is individual chromosomes that come from each parent, but as I explained above they will no longer be the same as the chromosomes they got from their parents.

 

[Ozymandyus]

Although your question has mostly been answered in various posts above, I thought I would do a point by point just so it's easy to read:

Sorry for being so incredibly stupid but here goes. I thought I was "a bit" across the sexual reproduction thing. But I was thinking about it and suddenly a massive void appeared and it all fell in. Here is my problem.

Woman:
Eggs are present from birth and are not "produced" fresh (like sperm are). The eggs should be either her fathers genetic compliment or her mother genetic compliment, from the split chromosome pairs that make her up. I was assuming her eggs are made from either 1/2 of the DNA strands that bonded when she was conceived.

When a woman makes eggs at birth, it is HER cells, the first of which was created during fertilization,that go through the process of meiosis: just as male sperm cells are made(it just happens earlier). Her 46 chromosomes, 23 of which are from her father and 23 from her mother, undergo 1 round of replication and 2 rounds of division, as you mentioned in your male sperm production. However, what you are missing is that during these divisions there is an independent assortment of chromosomes: they do not line up and reproduce the exact chromosome assortments of the grandfather and grandmother. Those 23 chromosomes could be 10 from her dad and 13 from her mother, or 1 from her dad and 22 from her mother, or any other combination.

In addition, during meiosis the chromosomes have some crossover, where the copies of her mother's and father's chromosomes can pair and form hybrid chromosomes: So, for example her dad's chromosome 10 and her mom's chromosome 10 could exchange some DNA which will make 2 chromosome 10s which are part her mom's and part her dad's.

This same independant assortment and crossing over happens during male gamete production, for the same results: new chromosome assortments that have half as many chromosomes as the original cells and have a different DNA composition than any of the other gametes.

Anyway, I didnt really go point by point but I hope it was clear enough and simplified enough to help rather than just restating what everyone else mentioned.

 

[simonecuttlefish]

Thank you to everyone who took the time to explain this. It does make sense now, and the cross distribution of genetic material is much clearer. I guess I was taking too much notice of the animations and not enough notice of what was being said.

Thanks heaps again.

 

[Aught3]

Found a short video which might help anyone still scratching their heads:



8 million! :shock: