I’d like to tell you a story. It’s the story of you. It’s an amazing story, and it begins, believe it or not, with a man named Gregor Mendel who was interested in peas.

Just like you, peas have both a mother and a father. What any pea plant looks like, how it grows, and lots of other details about the pea plant are a mixture of traits from mom and traits from dad. In Mendel’s time, no one really knew how that mixture worked, but they thought it was probably pretty complicated.

Mendel wondered if he could make some sense of the complexity by breaking down the problem into little pieces. Mendel knew that pea plants had some traits that were either this or that: either green or yellow peas, either tall or short stalks, either smooth or wrinkly peas, and so on. He decided to breed pea plants together (make them mom and dads) and keep track of exactly what he’d done, then keep track of the results, and see what he could see.

Let’s use just one of Mendel’s traits, say, color of peas. Mendel bred lots and lots of green pea plants together and always got green offspring. He bred lots and lots of yellow pea plants together and always got yellow offspring. Then he bred a green pea plant together with a yellow pea plant. What do you suppose happened? Did you guess that half the offspring were green and half were yellow? Or that all were a sort of greenish yellow mix? That’s what most people would have guessed. But that’s not what Mendel found.

Mendel found that the pea plants were ALL YELLOW!

You might be thinking, big deal, who cares, what’s the difference? But that’s not what Mendel thought. Mendel thought, that’s strange. Where did the green go? If green can disappear so easily, then why are there green pea plants at all?

So Mendel tried another experiment. He bred these new pea plants, the second generation, together. Remember, all these peas were yellow. You might think Mendel had done this experiment before. After all, he’d bred yellow pea plants with yellow pea plants again and again, and the result was always yellow peas. So why try again?

But Mendel did try it, and the results changed the world. Most of the peas were yellow. But around one in every four was green! Where did the green come from? It must have hidden from the first generation. But where did it hide? How did it hide? How could two yellow pea plants come together to make green?

What Mendel’s discovery showed us was that traits could be carried from generation to generation. The things doing the carrying, things which were completely mysterious to us, we decided to call genes. And the green pea experiment showed us something extraordinary. Genes are lumpy. They don’t get all spread out from generation to generation. Instead they can hide, somehow remaining intact even though we can’t see them, and appear again in later generations.

But what are these strange things called genes that can make pea plants tall or short, green or yellow, smooth or wrinkled, and can make my eyes brown or blue, my earlobes attached or dangly, and many, many other things as well?

Now the story shifts to another scientist, someone named Thomas Hunt Morgan. What’s interesting is that Morgan didn’t believe in genes. He thought the story told by Mendel’s experiments was too simple to explain the complex way that living things develop from mom and dad. But he was about to change his mind.

Morgan was interested in fruit flies. Fruit flies have red eyes, and Morgan was used to seeing red eye after red eye when he studied flies. But one day, Morgan found a fly with white eyes. It happened to be a male, so he crossbred it with a female red eyed fly to see what would happen. Just as in Mendel’s first experiment, Morgan found that all the offspring had red eyes. So just like the green peas, the white eye color in flies must be hiding. Morgan wondered what would happen when he bred this first generation of red-eyed flies together.

At first the results don’t look that surprising. Just as in Mendel’s experiment, Morgan found that most flies in the next generation had red eyes. About one in four had white eyes. But then Morgan looked at the number of male and female flies, and saw something amazing.

About half the flies were male and half were female. That’s what he expected. But of the male flies, about half had white eyes and half had red eyes. Among the female flies, there were NO white eyes. Not one. All the females had red eyes. What could that mean?

Morgan knew that there had recently been some exciting discoveries related to the chromosomes of fruit flies. ¬†Chromosomes are little strands found in the nucleus of every cell in the body – your body, fruit flies’ bodies, and the bodies of all other plants and animals. When cells divide, chromosomes divide, too, but nobody knew what the chromosomes did.

Fruit flies have either seven or eight chromosomes. But while female fruit flies have four pairs of chromosomes (for a total of eight), male fruit flies have only three pairs and a single, unpaired chromosome (for a total of seven). The working theory was that the extra chromosome made a fly female, while the missing chromosome made it male.

Now came Morgan’s amazing insight. If the trait (the gene) for eye color actually lived on that “sex” chromosome, then all his findings would make sense. Think of it this way. In the parents, the mother has two chromosomes that decide sex (let’s call them X chromosomes). The father has just one X chromosome. On the mother’s two X chromosomes lives the gene for red eyes. On the father’s one X chromosome lives the gene for white eyes.

Now think about the first generation. The female offspring get a red-eye gene from mom and a white-eye gene from dad. Since the white eye trait hides, all the female offspring have red eyes. The male offspring only get one X chromosome, and it comes from mom; dad doesn’t give them anything, which is what makes them male. And it gives them red eyes.

In the next generation, red-eye females and red-eye males breed. But the red-eye females are hiding that white-eye trait. All the females get an X from mom (with either the red-eye trait or the white-eye trait) and an X from dad (always with the red-eye trait), so they all have red eyes. But the males only get an X from mom. The result is that half the males have red eyes and half have white eyes.

It all works out, and as Morgan did more and more experiments with his flies he discovered that this simple explanation really did work. And so Morgan, who had been a doubter about the reality of genes, changed his mind. Genes are real, Morgan decided, and they live on chromosomes.

So what are these chromosomes? How are they put together, what are they made of, how do they divide, and most importantly, how do they carry the information that we call genes?

Chromosomes, it turns out, are made up of an amazing molecule called DNA. And DNA is the star of the next part of the story.