why can’t 2 eggs fertilize each other?

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I probably just have a really bad grasp on how genetics work, but if human eggs already have an X chromosome plus all the other necessary genes needed to combine with another haploid cell, why can’t 2 eggs just combine with each other to make an XX zygote?

In: Biology

(This is an ELI5, so don’t jump don’t my throat after making some generalizations for easy explanation please. )

Yes, each the egg and sperm carry half the DNA sequence to produce an offspring. However they aren’t the same. They are the opposite piece of the puzzle. Like having 2 left shoes don’t make a complete pair. That’s not to say with some intervention science may be able to flip and mirror one of those left shoes to effectively create a right shoe. For now, 2 lefts don’t make a pair.

Your body does some prep on sperm and eggs, basically “bookmarking” the DNA to make early development go more smoothly. The exact process of what is bookmarked is different between sperm and eggs. So if you try to fertilize an egg using DNA from two of the same type of sex cell, the bookmarks end up overall incorrect, it falls out of balance and can’t develop correctly. Cutting-edge research has figure out how to make egg-egg work in a lab with mice but the results aren’t great, and the male version has never been achieved.

Cells have a cell membrane.

When a cell takes something into it, the something can either get past the cell membrane and go into the cytoplasm, or it can’t.

If it can’t, then the cell kind of engulfs the something, surrounding it in cell membrane. (This is part of why it’s thought that mitochondria was a bacteria that was engulfed into a primordial cell.)

So since both eggs have a cell membrane, one couldn’t just enter into another one by going though it. It could theoretically be engulfed, where the inner cell is now surrounded by a second layer of membrane and making them into a massive cell with twice as much stuff. But either way, the DNA in the nucleus wouldn’t be able to reach the DNA in the other nucleus, and therefore they wouldn’t merge into an XX female zygote

It can in theory be done in a lab. Apparently Australian scientists have done it with two mouse eggs.

There are actually plenty of animals and plants that undergo what is called parthenogenesis, in which the female essentially clones herself by exactly the way you describe–two of her eggs fusing and forming an embryo. Sometimes, this is actually controlled by bacteria that infect the females. This happens with some wasps, for example!

While you could imagine this happening in a human, our eggs have lots and lots of control mechanisms to prevent this from happening. But if you were to artificially inject a nucleus from one egg cell into another, you could get a viable zygote that fully develops into an embryo. That is how we clone mammals today.

The simple answer is that there’s nothing about the DNA that prevents you from combining the chromosomes to make a zygote, in theory.

In reality, there’s a lot of complex chemistry and chain reactions that happens when a sperm fertilizes an egg, and that chemistry is what actually matters in turning the egg into a zygote. We don’t entirely understand that chemistry enough to replicate it in a lab without fertilizing an egg.