Unit Three. The Continuity of Life
10. Foundations of Genetics
In this pea pod, you can see the shadowy outlines of seeds that will form part of the next generation of this pea plant. While the seeds appear similar to one another, the plants they produce may differ in significant ways. This is because the gametes that produced the seeds contribute chromosomes from both parents, in effect “shuffling the deck of cards” so that a progeny plant will have some characteristics from one parent and some from the other. About 150 years ago, Gregor Mendel first described this process, before anyone knew what genes or chromosomes were. We now understand the process of heredity in considerable detail, and can begin to devise ways of treating some of the disorders that arise in people when particular genes are damaged in germ-line tissue. In this chapter you will watch as Mendel experiments with pea plants like the one above. Unlike researchers before him, Mendel carefully counted the number of each kind of pea plant his experiments produced and, looking at his results, saw a beautiful simplicity. The theory he proposed to explain it has become one of the key principles of biology.
10.1. Mendel and the Garden Pea
When you were bom, many things about you resembled your mother or father. This tendency for traits to be passed from parent to offspring is called heredity. Traits are alternative forms of a character, or heritable feature. How does heredity happen? Before DNA and chromosomes were discovered, this puzzle was one of the greatest mysteries of science. The key to understanding the puzzle of heredity was found in the garden of an Austrian monastery over a century ago by a monk named Gregor Mendel (figure 10.1). Mendel used the scientific process described in chapter 1 as a powerful way of analyzing the problem. Crossing pea plants with one another, Mendel made observations that allowed him to form a simple but powerful hypothesis that accurately predicted patterns of heredity—that is, how many offspring would be like one parent and how many like the other. When Mendel’s rules, introduced in chapter 1 as the theory of heredity, became widely known, investigators all over the world set out to discover the physical mechanism responsible for them. They learned that hereditary traits are instructions carefully laid out in the DNA a child receives from each parent. Mendel’s solution to the puzzle of heredity was the first step on this journey of understanding and one of the greatest intellectual accomplishments in the history of science.
Figure 10.1. Gregor Mendel.
The key to understanding the puzzle of heredity was solved by Mendel, who cultivated pea plants in the garden of his monastery in Brunn, Austria.
Early Ideas About Heredity
Mendel was not the first person to try to understand heredity by crossing pea plants. Over 200 years earlier, British farmers had performed similar crosses and obtained results similar to Mendel’s. They observed that in crosses between two types—tall and short plants, say—one type would disappear in one generation, only to reappear in the next. In the 1790s, for example, the British farmer T. A. Knight crossed a variety of the garden pea that had purple flowers with one that had white flowers. All the offspring of the cross had purple flowers. If two of these offspring were crossed, however, some of their offspring were purple and some were white. Knight noted that the purple had a “stronger tendency” to appear than white, but he did not count the numbers of each kind of offspring.
Gregor Mendel was born in 1822 to peasant parents and was educated in a monastery. He became a monk and was sent to the University of Vienna to study science and mathematics. Although he aspired to become a scientist and teacher, he failed his university exams for a teaching certificate and returned to the monastery, where he spent the rest of his life, eventually becoming abbot. Upon his return, Mendel joined an informal neighborhood science club, a group of farmers and others interested in science. Under the patronage of a local nobleman, each member set out to undertake scientific investigations, which were then discussed at meetings and published in the club’s own journal. Mendel undertook to repeat the classic series of crosses with pea plants done by Knight and others, but this time he intended to count the numbers of each kind of offspring in the hope that the numbers would give some hint of what was going on. Quantitative approaches to science—measuring and counting—were just becoming fashionable in Europe.
Mendel's Experimental System: The Garden Pea
Mendel chose to study the garden pea because several of its characteristics made it easy to work with:
1. Many varieties were available. Mendel selected seven pairs of lines that differed in easily distinguished traits (including the white versus purple flowers that Knight had studied 60 years earlier).
2. Mendel knew from the work of Knight and others that he could expect the infrequent version of a character to disappear in one generation and reappear in the next. He knew, in other words, that he would have something to count.
3. Pea plants are small, easy to grow, produce large numbers of offspring, and mature quickly.
4. The reproductive organs of peas are enclosed within their flowers. Figure 10.2 shows a cutaway view of the flower so that you can see the anther that holds the pollen and the carpel that holds the egg. Left alone, the flowers do not open. They simply fertilize themselves with their own pollen (male gametes). To carry out a cross, Mendel had only to pry the petals apart, reach in with a scissors, and snip off the male organs (anthers); he could then dust the female organs (the tip of the carpel) with pollen from another plant to make the cross.
Figure 10.2. The garden pea.
Because it is easy to cultivate and because there are many distinctive varieties, the garden pea, Pisum sativum, was a popular choice as an experimental subject in investigations of heredity for as long as a century before Mendel's studies.
Mendel’s experimental design was the same as Knight’s, only Mendel counted his plants. The crosses were carried out in three steps that are presented in the three panels in figure 10.3:
1. Mendel began by letting each variety self-fertilize for several generations. This ensured that each variety was true-breeding, meaning that it contained no other varieties of the trait, and so would produce only offspring of the same variety when it self-pollinated. The white flower variety, for example, produced only white flowers and no purple ones in each generation. Mendel called these lines the P generation (P for parental).
2. Mendel then conducted his experiment: He crossed two pea varieties exhibiting alternative traits, such as white versus purple flowers. The offspring that resulted he called the F1 generation (F1 for “first filial” generation, from the Latin word for “son” or “daughter”).
3. Finally, Mendel allowed the plants produced in the crosses of step 2 to self-fertilize, and he counted the numbers of each kind of offspring that resulted in this F2 (“second filial”) generation. As reported by Knight, the white flower trait reappeared in the F2 generation, although not as frequently as the purple flower trait.
Figure 10.3. How Mendel conducted his experiments.
Key Learning Outcome 10.1. Mendel studied heredity by crossing true-breeding garden peas that differed in easily scored alternative traits and then allowing the offspring to self-fertilize.