Once upon a time there was a monk named Gregor Mendel. He was born in 1822 in (what is now) the Czech Republic, in Europe. He came from a farming family and was very interested in not only the family business, but also in beekeeping. When he joined a monastery in order to become a monk, he was sent to college to learn more science.
While he was at the University of Vienna, he was inspired to perform a few experiments on pea plants. He was particularly interested in how it was that some pea plants were different from one another.
Mendel had a lot of time on his hands – and peas. So he separated the pea plants that he had into several groups. Two of these groups were tall pea plants and short pea plants. The tall ones were actually taller than him, about 6 feet tall! The short ones were only about a foot tall, so it was easy to tell which was which. The interesting thing that he had noticed, though, was that there were no pea plants that were neither one nor six feet tall. This observation meant to Mendel that there had to be something else going on inside the pea plant.
Remember, in 1822, nobody knew anything about DNA, genes or chromosomes. So, Mendel took his two groups of pea plants, the tall plants and the short plants, and separated them completely. From his work in beekeeping, he knew that bees could carry the pollen from one plant to another, so he made sure there was no way the tall plants could share pollen with the short plants. After a few generations, there was nothing but tall plants in the one group and nothing but short plants in the second group. Mendel had successfully made two pure groups of pea plants.
After this, he started a third group. He brought together the tall plants and short plants in this third group, making sure that every new plant was a combination of a tall and a short plant. What surprised him was that every single one of the resulting pea plants was tall! What was going on here? Wouldn’t you expect to see a mixture of tall and short pea plants?
Mendel needed to know more. He called these new tall plants the F1 generation (after the Latin for the first children). He wanted more information: specifically, he wanted to know what would happen when these tall plants had offspring with each other. Would they have all tall offspring? Was the shortness of the one parent completely lost?
Mendel then combined the F1 generation (the tall plants) with each other. The results were incredible: 75% of the offspring were tall, and 25% of them were short! Somehow, these plants had “remembered” their short grandparents – but how? So Mendel made a hypothesis that traits are carried from generation to generation in genes. Each individual has one copy from each of their parents, and the trait can either be dominant or recessive. In this case, he thought that the trait for being tall (for a pea plant) was the dominant trait and being short was recessive. But here’s the tricky part of what he figured out.
Mendel figured that each plant in the F1 generation got one tall trait from one parent and one short trait from the other parent. Since the tall trait (or allele) was dominant, then it hid the recessive trait for being short. Even though he didn’t use a Punnett square, he figured out that the cross between the two F1 plants went like this (T = tall, t = short):
T t T TT Tt t Tt tt
He repeated this experiment with many more traits and many more plants, coming up with roughly the same results each time.
Mendel looked at a total of seven traits of pea plants. One of the other traits was whether the seed of the pea was smooth or wrinkled. He observed that when he did the same experiment with this trait, the results were the same. So he decided to take the experiment one step further. He then combined the two traits: he took tall plants that made smooth seeds, tall plants that made wrinkled seeds, short plants that made smooth seeds, and short plants that made wrinkled seeds. He combined them in every way that he could think, but he found that no matter what he did, when he combined purely smooth seeds with purely wrinkled seeds, the offspring were all smooth. It didn’t matter if they were tall or short at all. After trying this with several more traits, he found a pattern. One trait didn’t affect any of the others. This came to be called the Law of Independent Assortment. All of the genes seemed to mix themselves up, and it didn’t matter what the other traits were.
Later, scientists came to find that things weren’t so simple. They actually saw that there were some traits that did depend on other traits. For instance, plants with yellow flowers were usually tall and plants with blue flowers were usually short. They explained this by saying that when genes are close together on a chromosome, they can sometimes show linkage. This means that some genes are “linked” together and are not independent.
There’s another type of linkage, called sex-linked traits, that Mendel did not describe. These traits are not necessarily traits that have anything to do with sex organs or sex cells. Traits that are sex-linked are on the sex chromosomes. It’s important to understand that male humans and female humans have one major difference in their chromosomes: the 23rd and final pair of chromosomes is “XX” in females and “XY” in males. The “Y” in males is actually just a small chromosome (see picture) and contains much less information than the “X” chromosome. Because of this, there are alleles on the X chromosome that are not on the Y chromosome. For the alleles that are on the X chromosome but not the Y, they will always show up, dominant or recessive! Examples of sex-linked traits include hemophilia and color blindness. These traits, since they can be dominated in a female but not a male, often show up much more often in males than in females.
Well after Mendel passed, other scientists looked at his work and figured out that it fit in with their theories of inheritance. In fact, it wasn’t until the 1930’s that Mendel was recognized for his efforts and people began to accept that genes could be responsible for evolution! But Mendel’s work still didn’t explain quite a few things about genetics.