What is produced from the combination of genetic material from the egg cell and the sperm cell multiple correct answers here?

You can’t make a baby with two moms by simply fusing two eggs or adding one egg’s DNA to another’s.  Even though the resulting embryo would have the usual 46 chromosomes, this wouldn’t work.

The reason isn’t some special string of A’s, G’s, T’s or C’s found in dad’s DNA.  No, instead it has to do with chemical marks found on egg and sperm DNA.  This methylation (as the marks are called) makes the DNA from each parent unique, and you need both to make a baby.

What these marks do is affect how at least 80 different genes are used.  In science speak, these genes are imprinted.

As you may remember, we have two copies of each of our genes, one from each biological parent.  The chemical marks shut off either the copy from the egg or sperm, depending on which has the marks present.. 

If an embryo’s DNA came from two parents of the same sex, then both copies of some of these imprinted genes will be shut off.  And for the rest of these genes, both copies will be turned on.  Embryos simply can’t survive when so many genes are out of whack. In fact, diseases like Angelman syndrome, Prader-Willi syndrome, and Beckwith-Wiedemann syndrome can happen when just one of these genes isn’t passed on properly. 

DNA from eggs and sperm are different because of chemical marks, like the methyl groups shown here. Via Wikimedia Commons

In the animal world, this weird little system of imprinting seems to be peculiar to all mammals except the egg laying ones like the platypus.  (Imprinting also happens in flowering plants.)  Other animals don’t imprint their DNA, so why do mammals?

There are a bunch of theories but one I like is that imprinting prevents something called parthenogenesis.  This is a long word for a process where an egg doubles its DNA on its own and grows into a new beast without DNA from a sperm.  Lots of other animals can do this, but mammals can’t.  Most likely this is because imprinting prevents it from happening.

Of course for your question, the why of imprinting doesn’t matter… we only care that it is here and keeps us from fusing two eggs to make a baby.  But don’t give up so easily.  There may be workarounds.

One possibility, which has worked in mice, is to make one of the egg’s DNA look a bit more like a sperm’s.  Another would be to coax one mom’s cells into becoming sperm.  Or, if we want two dads instead, coaxing one of dad’s cells into becoming an egg.

As I said, an obvious way around the problem of imprinting is to tweak one mom’s DNA so it looks more male.  This is as hard as it sounds.  Still, researchers managed to get two female mice to produce offspring back in 2004 by doing something very similar to this. 

The genome of this black mouse originated from two eggs - no sperm! Via Wikipedia

They started off using a mutant mouse whose females had DNA that was more male-like than typical.   But they found this wasn’t enough to make offspring when combined with a second mouse’s egg.

To make offspring, the egg from the mutant female had to be combined with an immature egg from a second female.  This immature egg hadn’t yet added marks to all of its DNA.

Even though the researchers got this to work, it was far from perfect.  They only managed to get two daughter pups out of 457 tries1. 

Now obviously this wouldn’t work for people.  We can’t mutate one mom and then have hundreds of miscarriages before we get one daughter.  Things have improved in mice since then but not enough to move it into people just yet.

To get a biological child with same-sex parents, we’ll probably have to get really creative.  One way might be to use stem cells.

Imagine two women who want to have a child together.  We’ve talked about why this is so tough to do. 

But what if instead of trying to make the DNA in one woman’s egg more like a sperm’s, we coaxed some of her cells into actually becoming sperm?  Or what if we could coax male cells into becoming eggs?  Now two men could have a baby too

We could try reprogramming female cells to become sperm. Via Wikipedia

This isn’t coming to a fertility clinic near you any time soon, but it might be possible one day.  The way we might do this is to take one parent’s cells, turn them into the equivalent of embryonic stem (ES) cells, and then coax them into becoming sperm or eggs.

Scientists are getting pretty good at turning cells into those magical ES cells that can become any other kind of cell.  So far, though, no one has managed to make sperm or egg cells with artificially created ES cells. (Scientists call these artificially created ES cells induced pluripotent stem cells or iPS cells to distinguish them from natural ES cells.)

There are some signs of progress though.  Scientists have been able to turn natural ES cells from a male mouse into a sperm cell that can fertilize an egg.  This fertilized egg can even then go on to become a happy, healthy pup. 

Scientists have also done similar things to create precursor egg cells from natural ES cells from a female mouse. It should only be a matter of time before they get these working with human ES cells too.

Of course this isn’t enough to do what we want yet.  We first need to try to get this process to work with iPS cells. Scientists have made sperm and egg precursors from mouse iPS cells but, to my knowledge, they haven’t  yet created a fertilized egg from them.

So far all of this has focused on making eggs from female cells and sperm from male cells.  This will be a great boon to couples dealing with infertility, but getting male cells to become an egg or female cells a sperm will probably be much harder.  Perhaps not impossible, but not as “simple” as what we’ve talked about so far.

Male and female cells are fundamentally different.  Yes, turning them into ES cells should eliminate the original markings but we can’t know for sure until we try. 

There may be a whiff of maleness or femaleness left in our artificially created ES cells. And as we’ve seen, it only takes a whiff to cause a genetic disease.

But still, we may one day be able to pull off what you suggest.  Then, same-sex partners might be able to have biological children.

Great question! Identical twins share pretty much the same DNA, so your question is also like asking the odds of a woman who makes twice as many eggs creating the same egg twice. 

The answer is that it is extremely unlikely. Not impossible, but the odds are very much against two eggs having the exact same DNA.

Because of recombination, the odds are very much against these identical twins ever making identical eggs.

Our DNA is stored in the form of 23 pairs of chromosomes. In a process called meiosis, one chromosome from each pair is put into an egg. We’ll talk about this more below, but it’s random which chromosome is selected.

If this was all there was to it, then we’d just need to figure out how likely it is to pick the same 23 chromosomes from each pair. Since each pick is random, this is the same as the chance of flipping a coin 23 times and getting heads each time.

The math here is (1/2)23, which comes out to a 1 in 8,388,608 chance. Seems like a really small chance, but when we are dealing with four million eggs (around two million from each twin), it is more likely than we might think.

But this isn’t all there is to passing on our DNA. Through a process called recombination, DNA is randomly swapped between each pair of chromosomes. 

What this means is that the odds are actually way worse than 1 in 8,388,608. In fact, there are close to an infinite number of possible ways for our DNA to mix on its way to eggs or sperm. Which means it is nearly impossible for two eggs from identical twin women to have the exact same DNA.

Eggs begin their life as germ cells. Germ cells are cells that are set aside very early in embryonic development to make eggs or sperm. 

During embryonic development, germ cells go through a series of mitotic divisions. Mitosis is where all of the DNA in the mother cell is copied and passed on to two daughter cells. This produces a lot of germ cells. 

If the embryo is male, the germ cells will continue to divide mitotically to give rise to sperm during adulthood. Men are constantly making new sperm through their whole life.

However, if the embryo is female, the germ cells will stop dividing mitotically before birth. Women are born with all the eggs they will ever have*.   

To become eggs (or sperm), the germ cells must undergo another kind of division, called meiotic division. Instead of giving each daughter cell a copy of all the mother cell’s DNA, meiotic division gives each daughter cell a copy of only half the mother cell’s DNA.

This way, when the egg and sperm meet, the fertilized egg will have the right amount of DNA: 23 chromosomes from mom and 23 chromosomes from dad.

* Although there is scientific debate on whether a few germ cells are still dividing during adulthood in women. Click here to learn more.

As I said earlier, our DNA is stored in the form of 23 pairs of chromosomes. Each pair is made of one chromosome from mom and one chromosome from dad.

What this means is that mother germ cells contain two copies of each of the 23 chromosomes. So, to divide up her DNA, the mother cell could give each daughter one copy from each of her 23 chromosome pairs.

But, meiosis isn’t that simple. There are two parts to meiosis: meiosis I and meiosis II.

To make things a bit simpler, we’ll follow a single pair of chromosomes through its meiotic journey. Remember, the other 22 pairs of chromosomes are all doing the same thing. Here is the pair we will deal with:

I have made one chromosome red and the other blue. This will make it easier to follow what is going on.

During meiosis I, the mother cell copies its DNA so that there are twice as many chromosomes (4 copies of each chromosome versus the original 2). Each new chromosome is still connected to the old chromosome (forming an “X” shape).

Here is what that might look like:

In the next step, the chromosomes undergo recombination so that the DNA is shuffled between different copies of the same chromosome.

Here is one way this might turn out:

Now, let’s split them apart to get a better look at them:    

As you can see, none of the chromosomes is exactly the same as either of the original pair. The first two here have a bit of blue mixed in with mostly red and the last two have a bit of red mixed in with mostly blue.

In real life, the chromosomes are split apart during meiosis II. And humans only make one egg per mitotic cycle, so only one of these chromosomes ends up in the egg. The other three chromosomes get excluded from the egg and are called polar bodies. (If we were discussing sperm formation, the four chromosomes would end up in four separate sperm.)

Here is what that might look like:

The way I have drawn this is just one case of how recombination could have happened. There are actually, many, many different ways to swap DNA. Here are two of an almost infinite number more:

And this is just for one chromosome! Now imagine the same process happening with the other 22 pairs of chromosomes.

I think you can see why it would be so unlikely that identical twin women would create identical eggs, or that one woman could make the same egg twice. And why two people who aren’t identical twins are never exactly the same.