Mendel Got Extremely Lucky (...or Maybe He Lied)

Thank you to Brilliant for supporting this SciShow video. You can keep building your STEM skills at Brilliant.org slash SciShow with 20% off an annual Premium subscription. Gregor Mendel and his pea plants are the bane of many a middle school science student learning genetics for the first time. But Mendel himself may have been one of the luckiest people ever to put pen to paper in the name of science. Not to say he didn't work incredibly hard for many years to make the discoveries he did. It's just that his chosen organism, the common pea, was about as good as it gets for research on genetic traits. And as a result, Mendel and his lucky peas helped build the field of genetics as we know it. But if he'd chosen other pea traits to focus on or another model organism altogether, he would have had his work cut out for him. Because it turns out, those laws have more exceptions than not. Mendel's work was a huge step in explaining how certain traits are passed from one generation onto the next. Things like blood group or eye color. Though, when Mendel started this work way back in the 1850s, he wasn't actually looking at human traits, but traits of the common pea, Pisum sativum. He selectively bred pea plants and showed how a dominant characteristic can be passed down and remain visible across generations. On the other hand, recessive traits may seemingly skip generations, but pop up in the future. The recessive trait is still being passed along over generations, but they're typically overshadowed by dominant traits, so they don't show up as observable traits or phenotypes. His work centered around seven observable traits in pea plants, like plant height, seed shape, pod shape, and pea color. And most of these traits had two forms. For example, smooth or wrinkled-shaped seeds. That simplicity is important. Introductory textbooks even today might present traits as having one dominant and one recessive form. But remember that eye color and blood groups come in more than two flavors. These traits and their binary level of simplicity allowed Mendel to work out the law of segregation. That each parent organism passes down a single discrete unit for a trait, which we now call a gene, to its offspring. His pea breeding also led him to come up with the law of independent assortment. That traits are passed down independently, and don't influence one another's inheritance. So, whether or not a seed was smooth or wrinkled didn't influence if it was also yellow or green. Together, these made up Mendel's principles of inheritance. Of course, Mendel's work on these pea traits happened long before we even knew what genes actually were. So, kind of a big deal. His findings basically let us understand that genes retain their identity across generations, and don't fuse or blend together over time. As in, a pea with smooth seeds can have offspring with wrinkled seeds, because the wrinkled gene is still in there to be passed along for a future generation to express. But he actually got super, super lucky with his peas, because those rules have a lot of exceptions. It wasn't until the early 1900s that his ideas got some traction, and people started conducting experiments to flesh them out further. The geneticists William Bateson, Edith Rebecca Saunders, and Reginald C. Punnett found that not all of their pea plant crosses followed Mendelian rules. Some traits appeared more frequently than Mendel's ratios would predict. Which would only make sense if they were tagging along with other traits, if they weren't sorting independently. As a result, the group concluded that some traits were likely coupled together. Although, they weren't totally clear on how. And other researchers filled in the gaps, including Thomas Hunt Morgan. He used fruit flies to study genes that broke Mendel's rules of independent assortment. In his experiments, he bred fruit flies with white eyes instead of the normal red. But strangely, he didn't breed any white-eyed females, only males. This made him wonder if the eye color trait was only being passed down by males. His contemporaries had recently worked out that sex in flies was determined by the inheritance of a specific chromosome. Female flies get two X chromosomes, while males get one. So he explored the idea that not only was the white eye color recessive, it was also linked to the X chromosome. That's why it wasn't showing up in the first generations of females he was breeding, since they were all still getting a dominant red-eyed gene from their maternal side. That masked any white-eyed genes being passed along. It wasn't until Morgan crossed white-eyed males with a previous generation of their own female offspring that white-eyed females finally popped up. That's thanks to their set of both paternal and maternal white-eye color-linked X chromosomes. Morgan's work provided evidence for the chromosome theory— the idea that genes travel in groups, on chromosomes. This experiment showed that traits unrelated to sex can still be linked to sex chromosomes. But it also neatly explains why traits can travel together, in general. In Mendel's case, he lucked out, because his choice traits were mostly found on separate chromosomes. So genetic linkage wasn't factoring into his data. That allowed him to come up with a fairly simple pattern of inheritance, and he just let everyone else fill in the complicated bits. Basically, you could say that Mendel walked so that later scientists could run. He got lucky in finding a model organism that showed off the rules of inheritance in the simplest way possible. Well, some people have raised the question of whether it was just luck, or if he did a little massaging of the evidence to make his point clearer. Regardless, in laying out the rules, he gave later scientists the framework they needed to understand how the exceptions were working. Which is really what any scientist hopes to leave for the next generation. And 200 years out, we're still talking about it. People call Mendel the father of genetics. His pea plants laid the foundation for the genomics investigations we have today. And Brilliant's courses on computational biology has an entire section about genomics that goes into genetics, genotyping, and how big a genome is. In this hands-on course, you'll answer the easy questions like, What is life? before diving into real data from African ancestry maps and Viking colonization across the northern Atlantic. The people at Brilliant believe that STEM knowledge can help you achieve your personal and professional goals, so they've created interactive courses to spark curiosity. And since nobody is an expert in everything, they partnered with quantitative biologists and biophysicists at Caltech and Duke University to make the computational biology course. So, to start learning for free, click the link in the description down below, or visit Brilliant.org slash SciShow. There, you'll get 20% off the annual Premium subscription. Thank you to Brilliant for supporting this video, and thank you for watching.