[Intro]

Ever seen a crazy looking butterfly or a bird that looks like two different individuals smashed together? Not like a griffin or a centaur with two different species, but two members of the same species that have different markings.  Like a butterfly with two different wing patterns; or a lobster that's red on one side and brown on the other. You probably haven't because they're rare, but they ARE out there.

They're know as gynandromorphs. These organisms have both male and female characteristics, often split right down the center of their bodies. The condition can be found in about 1 in 10,000 butterflies and other insects, but also in crustaceans and other arthropods.

So: how does such a thing happen? Well, all sexually reproducing animals begin as a single fertilized egg, or zygote, that divides itself again and again through mitosis. One cell turning into two, and two turning into four, and so on, to make a baby animal. Now in mammals, those divided cells differentiate themselves later on in development, so when they first split, they can turn into pretty much any tissue.

But in arthropods, all of those early cell divisions are determinate. That means that every cell has a particular inflexible destiny from the very beginning. So, for example, when a butterfly zygote first divides, those first two resulting cells will each determine the left and right sides of the animal. The second division determines what will be the front and the back; and the third assigns top and bottom, and so on.

Now, another thing to keep in mind is that before each cell splits, it has two copies of the organism's DNA coiled up in pairs of chromosomes: one set for each new cell. And that information includes the organism's sex- determined by chromosomes we call X and Y. In butterflies, XY is the female, and XX is the male. And yes, that is the opposite of how it is in humans.

If there happens to be a mutation in one of these sex chromosomes, usually the information can still be read - like if an animal ends up with just one X and no Y, it'll be female; if it gets three X's, it's male.

But if something goes wrong during the earliest stages of mitosis, and the sex chromosomes don't properly separate, one of the new cells may have too many chromosomes, while the other doesn't have enough.

So, if this happens during the development of, say, a male Swallowtail butterfly - which would normally be XX - it might end up with one cell only having one X, making it female, and then other cell with three X's, making it male.

Then, all the subsequent divisions from those two cells would be all male and all female, and you'd get a bilateral gynandromorph. One that looked like a half male and half female stuck together.

Now, if the error occurs in later division, you might end up with a more patchy mix or a mosaic of male and female characteristics.

Gynandromorphy is not the same thing as hermaphroditism, which is when an organism has both male and female sex organs. Depending on when the error occurs - and what the anatomy of the animal is like-, gynandromorphs are not necessarily hermaphroditic, although no matter what, it's likely to be sterile.

And, as far as we know, gynandromorphy isn't possible in humans or other mammals because our ovaries and testicles release specific hormones that promote either female or male traits. These dominant hormones can actually override abnormalities in our sex chromosomes.

Consider it just one more way in which sexual reproduction and just life on Earth in general can be downright weird.

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