7.3: Mendel learned about heredity by conducting experiments.

What do parents “give” their offspring that confers similarity? It wasn’t until the mid-1800s that any real headway was made on answering this question. At that time, Gregor Mendel, a monk living in what is now the Czech Republic, conducted studies that not only shed light on this question but practically answered it completely. Mendel understood the essence of the genetics puzzle and set out to piece it all together.

When Mendel turned to questions about heredity, there were no obvious answers. One idea that had been popular since the late 1600s suggested that an entire pre-made human—albeit a tiny one—was contained in every sperm cell (FIGURE 7-7). Though imaginative, this idea could not explain why children resemble both their mothers and their fathers, not just their fathers. Another popular idea suggested that offspring reflect a simple blending of their two parents’ traits by blending of the blood. But how then, could brown-eyed parents give birth to blue-eyed children?

Sometimes scientific breakthroughs are made because a new technique is invented or a lucky observation is made by chance. Neither played a role in Mendel’s success. He didn’t do anything radically new, but simply applied the tried-and-true process of methodical experimentation and scientific thinking (as described in Chapter 1). In particular, three features of Mendel’s research were critical to its success (FIGURE 7-8).

• 1. He chose a good organism to study: the garden pea. It’s not that Mendel had a particular fondness for vegetables. Moreover, his goals were to understand inheritance in all organisms, not just plants. But cats and dogs or even mice wouldn’t have served his purposes very well, because they would have been too hard to take care of in the large numbers he required—thousands and thousands of individuals. Humans, too, would have made a terrible study organism. We take too long to breed (and won’t produce offspring on command). Pea plants, on the other hand, were simply the right tool. They are relatively easy to fertilize manually by “pollen dusting.” A single cross—the process in which male pollen (carrying sperm) is used to fertilize eggs—produces numerous offspring. In addition, pea plants reproduce quickly, so Mendel could conduct experiments that included multiple generations.
  
• 2. Mendel chose to focus on easily categorized traits like shape and color. For instance, all peas of the variety that Mendel studied are either round or wrinkled in shape, with nothing in between. In addition, all peas are either yellow or green in color, never any intermediate shade. In all, Mendel looked at seven different traits of the pea plants, but for each trait only two variants ever appeared. Mendel and his research assistants could easily observe and unambiguously identify the trait variants.
• 3. Mendel began his studies by first repeatedly breeding together similar plants until he had many distinct populations, each of which was unvarying for a particular trait. He described these plants as true-breeding for that trait because they always produced offspring with the same variant of the trait as the parents. For example, when true-breeding round-pea plants were crossed together, they always produced plants with round peas. True-breeding purple-flowered pea plants always produced purple-flowered offspring, while true-breeding white-flowered pea plants always produced white-flowered offspring. It took a lot of prep work to establish these populations, but once Mendel had them, he was in a position to set up all of the different crosses that enabled him to piece together the genetics puzzle.

Once he had obtained his groups of true-breeding plants, Mendel began a straightforward process of experimentation. He crossed plants with different traits—for example, he crossed plants with green peas and plants with yellow peas—and observed large numbers of their offspring, counting the number of plants that produced green peas and the number of plants that produced yellow peas. (This cross, written as “green × yellow cross,” always resulted in plants that produced only yellow peas.) Mendel devised a hypothesis that would explain his observations and generate predictions about the outcomes of further crosses. Then he conducted those crosses to see whether the predictions generated by his hypothesis were borne out.

There was a simple elegance to Mendel’s work: from his predictions he articulated simple questions that could be answered by his experiments. For example, “If I cross a plant that produces wrinkled peas with a plant that produces smooth peas, will the offspring plants produce wrinkled or smooth peas?” His crosses would come out either one way or the other. Everything was black and white. This process of performing well-thought-out, rigorous experiments was a radical innovation for biology. We explore his results in the next section and see that they allowed Mendel to clearly and definitively describe how the traits he studied were inherited.