Chapter 8: Genes
The next question that needed to be answered was: What gave plants and animals their characteristics?
This was discovered by a man named Gregor Mendel. Gregor Mendel worked in a monastery in the country of Austria-Hungary in the 1800s. (Austria-Hungary no longer exists—now Austria and Hungary are two different countries, and the rest of Austria-Hungary has broken up into many other countries.)
Gregor Mendel grew pea plants in the garden of the monastery. He found that most of the plants grew peas that were smooth, but about 1 out of every 4 grew peas that were wrinkled. So he tried breeding different plants with each other to see what would happen.
When he bred plants with wrinkled peas with other plants with wrinkled peas, he always got more plants that had wrinkled peas.
When he bred plants that grew smooth peas, some of them would always create more plants that had smooth peas.
When he bred other plants that grew smooth peas, they would sometimes create more plants that grew smooth peas, and sometimes create plants that grew wrinkled peas.
When he bred plants that grew smooth peas with plants that grew wrinkled peas, two different things could happen. One out of three plants that grew smooth peas would produce a second generation that always grew smooth peas. Two out of three plants that grew smooth peas would produce a second generation where half of the plants would grow smooth peas and half would grow wrinkled peas.
If a plant grew smooth peas, but he knew that one of its parents grew wrinkled peas, he found that if he bred the plant with another plant that grew wrinkled peas, half of the next generation would grow smooth peas, and the other half would grow wrinkled peas.
If a plant grew smooth peas, but he knew that one of its parents had grown wrinkled peas, and he bred it with a plant that grew smooth peas and whose ancestors had always grown smooth peas, the plants of the next generation always grew smooth peas.
Gregor Mendel discovered a lot of clues. Then he tried to find a pattern of cause and effect that could make all the clues fit together.
You know that if you want to make tortillas, you can follow a recipe. You can grind up corn, mix it with water, mix it with some other things if you want, and then heat it over a fire. If you mix different ingredients together, you get different tortillas.
In the same way, Gregor Mendel discovered that peas, and all other plants and animals, grew the way they did because they had some sort of recipe inside them. The recipe for each pea plant included a part that made its peas either smooth or wrinkled. There were two parts to that recipe. When he bred two pea plants together, each of them passed half of their recipe on to each of their children. Each pea plant inherited half of its recipe from each of its parents, so that each pea plant got a whole recipe.
If both halves of the recipe said wrinkled peas, the plant got wrinkled peas. If both halves of the recipe said smooth peas, the plant got smooth peas. If half the recipe said smooth peas and the other half said wrinkled peas, the plants got smooth peas.
That makes four possibilities: The plant gets a smooth recipe from both parents, it gets a wrinkled recipe from both parents, it gets a smooth recipe from its father and a wrinkled recipe from its mother, or it gets a wrinkled recipe from its father and a smooth recipe from its mother.
That makes all the combinations of characteristics Gregor Mendel observed. Out of those four combinations, one makes plants with wrinkled peas, and the other three make plants with smooth peas. Out of those three plants with smooth peas, one will pass on a recipe for smooth peas to all of its children. The other two will pass on a recipe for smooth peas on to half their children, and recipes for wrinkled peas to the other half of their children.
Now we can go back to talking about plants and animals being made up of chemicals.
Every plant and animal is made of a lot of pieces that are so small you need a microscope to see them. These are called cells. Cells are basically like the bricks that make up a building. Every part of a plant or animal’s body is made of different cells. Animals have bone cells, skin cells, blood cells, hair cells, muscle cells, liver cells, and so on. Plants have bark cells, wood cells, leaf cells, and so on. Each of them work differently, so I won’t bother to try to tell you how they all work. That’s something else you could learn from teachers or university students in your area.
What all of an animal or plant’s cells have in common is that each of them contains the recipe for the entire animal or plant. All of the recipes are made up of big, complicated molecules.
Some of the smallest of these molecules are protein molecules. There is a lot of protein in meat and beans. When you eat meat or beans, this is what you’re eating.
I’ve told you how everything in the world is made up of 100 kinds of atoms, and those 100 kinds of atoms are made of 3 kinds of subatomic particles. Here we see something like that again, because every living thing in the world is made up of the same 4 kinds of protein molecules. In the same basic way that everything in the world is different because of the different ways those 100 kinds of atoms and 3 kinds of subatomic particles interact with each other, every living thing is different because of the different ways those 4 protein molecules interact with each other.
Those protein molecules bond together to form even bigger molecules. The easiest way I can explain the next part is to say these bigger molecules are called genes. This isn’t technically true, but it’s close enough to true for you to understand how it works. Some genes might be parts of molecules, and other genes might be made up of more than one molecule. The point is, genes are the next bigger unit after proteins, and they work like molecules, even if they aren’t literally molecules. If you want to go to college and learn about biology you’ll have to learn something a little different, but if you’re farmers, that’s close enough.
Gene molecules bond together to form bigger molecules, called chromosomes. There are many chromosomes in each cell. Humans have 46 chromosomes in each cell. Other animals and plants have different numbers.
All of the chromosomes are divided up into pairs. Humans have 23 pairs of chromosomes. All humans have the same 23 pairs of chromosomes, and the chromosomes all work the same basic ways.
Chromosomes are also called DNA molecules. A DNA molecule is a chromosome. All of your DNA is divided up into 46 DNA molecules.
Genes are what create the different characteristics of plants and animals. Genes fit together to make the same chromosomes, but the genes that can fit into the same part of a chromosome are slightly different from each other, so they work slightly differently from each other.
If you look at your arm, you can see that you have five fingers, then you have a hand, then you have a wrist, then you have a lower arm, then you have an elbow, then you have an upper arm, and then you have a shoulder. If you look at your friend’s arm, you can see that he has five fingers, a hand, a wrist, and so on, just like you do. His fingers, hand, wrist, and so on are different from yours, but they still work the same basic ways. If you had his fingers on your hand, and he had your fingers on his hand, your hands would work a little differently, but your hands would still work pretty much like they do now.
If you look at a horse, you can see that where you have your arm, the horse has a hoof, a foot, an ankle, a lower leg, a knee, an upper leg, and a shoulder. If you look at another horse, you see the same thing. Again, their hooves, feet, ankles, and such work the same way but are slightly different from each other.
When a woman has a baby, you know that the baby is going to be born with arms pretty much like yours. It isn’t going to be born with the forelegs of a horse where its arms are supposed to be.
Chromosomes and genes work the same way. A chromosome is like your arm, because it has the same pieces in the same places, and all the pieces work the same basic way.
Genes are like the pieces of your arm. They’re the same basic pieces, but they’re a little different from one person to another. One person might have longer fingers than another person, or one person might have bigger hands than another person. That makes your arms work a little bit differently, but still pretty much the same way. The difference between your arm and your friend’s arm is a lot less than the difference between your arm and the foreleg of a horse.
Human genes make human chromosomes, and horse genes make horse chromosomes. Human chromosomes make the full recipe for a human because they get all the right pieces in all the right places. Likewise for horses.
Human are a little different from each other because they have different genes, but they are pretty much the same because they all have human genes. Each human is unique, but each human has more in common with any other human than he or she has in common with a horse.
(I should say here that humans and horses actually have a lot of the same genes, which is why humans and horses have the same basic shape of a head, a body, four limbs, similar bones, muscles, skin, hair, blood, internal organs, and so on. Cows, pigs, chicken, cats, dogs, and mice all have that basic pattern of a head, a body, four limbs, and similar bones, muscles, skin, hair, blood, and internal organs. The differences between humans and horses and any other animal are caused by differences in a fairly small number of genes, compared to all the genes they have. This is another one of those things that you’ll have to learn more about if you go to the university to learn about biology, but as far as farmers are concerned, humans have human genes and horses have horse genes.)
Genes that each can fit into the same place in a chromosome and each do the same basic job are called alleles of each other. The difference between your hand and your friend’s hand is caused by the different alleles. You both have hand genes that work the same basic way. Instead of saying that you have different genes for your hand than your friend does, and saying that you have different genes for your hand than you have for your fingers, now you can say you have different hands because you have different alleles of the same hand genes. That just makes it easier to keep track of what we’re talking about.
(Again, I’m simplifying this a little. You don’t actually have genes that create your hands, you have many genes that work together to create your hands, and some other genes that create your bones, some that create your muscles, and some that create your skin. Most parts of your body are created by lots of different genes interacting with each other. As far as genes relate to you, I can say you have hand genes, but if you went to a university to learn about biology, you would have to learn something more complicated.)
When Charles Darwin discovered that characteristics are passed down from parent to child, and the characteristics that let a plant or animal survive best in its environment get passed on to the most members of the next generation, what he discovered—without realizing it—was that different alleles of the same genes that gave the plants or animals their characteristics were the things that were being passed down from parent to child.
When the animals split up between living in the hills and living on the plains, they each had genes that created their leg muscles. Since the ones who lived on the plains could run fast on the plains, and running fast helped them survive and reproduce, the ones who had alleles of the leg muscle genes that gave them strong leg muscles, survived and reproduced the best. They had the most children, so they passed more of their alleles down to the next generation. By passing their alleles down to the next generation, they passed their characteristics down to the next generation—because the alleles were what gave their children their characteristics.
Eventually, those original animals, whatever they were, kept certain alleles in their group, while the others died out. That combination of alleles that survived and spread through the group gave that group of animals the characteristics that made them horses.
Meanwhile, the animals that came from the same ancestors but moved into the hills, where they needed different characteristics, spread a different combination of alleles through the group from generation to generation. When that combination of alleles spread through the whole group, and the other alleles died out eventually, those animals developed the characteristics that made them donkeys, which made them different animals from horses.