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?

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Question 7.2

Individuals who share common ancestry are called “blood relatives,” yet they don’t actually share any blood. How might the phrase “blood relatives” be a reflection of early conceptions of inheritance?

Figure 7.7: The “pre-made human.” A tiny human in every sperm cell? Why would children have any resemblance to their mothers?

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).

Figure 7.8: Mendel’s pea plant experiments. Careful planning and a well-selected study organism contributed to the success of Mendel’s experiments.

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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.

TAKE-HOME MESSAGE 7.3

In the mid-1800s, Gregor Mendel conducted studies that help us understand heredity. He focused on easily observed and categorized traits in garden peas and applied methodical experimentation and rigorous hypothesis testing to determine how traits are inherited.

Why were pea plants a good choice for Mendel to study heredity?

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