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Meticulous work solves mystery of inheritance
Why does the proverbial apple “not fall far from the tree?” Why do people typically resemble their parents? Why are illnesses often passed down from one generation to the next?
Today we know that genes carry traits from parents to their offspring, but let’s go back to a time in the mid-1800s before genes had been discovered, when Gregor Mendel was working to unravel the riddle of heredity.
This Augustinian friar was not the first to explore these issues. Prior to Mendel’s groundbreaking work, farmers and horticulturalists had been breeding plants and animals for thousands of years, but this work was not sophisticated or scientific. Also, philosophers, naturalists and scientists had tried to reveal patterns of heredity by positing theories, crossbreeding animals and experimenting with plants.
Nevertheless, Mendel was the first to conduct broad, thorough, systematic and sufficiently rigorous experiments to discern any universal laws governing inheritance. How did he do it?
Peas in a Pod: Tracking Traits
Mendel began his famous experiment, which lasted eight years, with careful observation and planning. He selected the garden pea, Pisum sativum, due to several desirable characteristics: its seeds were readily available; the plants are easy to grow; pollination is relatively straightforward and controllable; and several physical characteristics, or traits, of the garden pea are easy to distinguish.
Mendel selected the following clearly contrasting, heritable characteristics to study:
• Seed color (yellow or green)
• Seed shape (smooth or wrinkled)
• Pod color (yellow or green)
• Pod shape (inflated or pinched)
• Flower color (purple or white)
• Flower position (axial or terminal)
• Stem height (tall or short)
For two years, the scientist grew generation after generation of pea plants to develop true-breeding lines for each of these 14 characteristics. A true-breeding line is one that always produces one particular trait, such as yellow seeds. If green seeds showed up in this line later during the experiment, Mendel could be sure that it would be the result of his intervention in the pollinization process.
A common misconception about heredity during Mendel’s time was that characteristics blended, producing, for example, medium-high stems from crossing a true-breeding line for tall stems with a true-breeding line for short stems. Some scientifically inclined breeders had noticed characteristics disappearing in one generation and reappearing in another, but they did not take their observations any farther. Mendel followed these traits through enough generations to find that all the inherited traits he was studying are distinct and remain intact. They do not blend together during sexual reproduction; instead, they appear, disappear and reappear from generation to generation.
In addition to determining that the traits were discrete, Mendel discovered that their appearance followed certain patterns. Some were more common than others. For example, when Mendel bred a true-breeding tall plant with a true-breeding short plant, all the offspring were tall. Therefore, he determined that the trait for tallness is dominant and the trait for shortness was recessive, at least concerning garden peas.
Remarkably, all seven sets of characteristic traits he studied behaved in the same way in the first generation of offspring. Yellow seeds, smooth seeds, green pods, inflated pods, purple flowers, axial flowers, and tall stems were all dominant traits. Meanwhile, their corresponding alternative characteristics were all recessive.
Mendel meticulously counted all the plants’ characteristics in the second generation of offspring and noted a three-to-one ratio of dominant to recessive traits. Since Mendel was also a mathematician; he recognized a pattern in this ratio.
Mendel concluded that every characteristic he was studying must be controlled by two “elements” that are present in every pea plant. As part of sexual reproduction, these elements separate and only one is passed down to the offspring.
Thus, if a plant from a true-breeding line for a dominant trait (with the gene pair AA) was crossed with one from a true-breeding line for a recessive trait (with aa genes), all the offspring would manifest the dominant trait because the only possible combination of genes would be Aa. In the second generation of offspring, however, crossing a plant with Aa genes with another plant with Aa genes would reveal four possible gene combinations in their offspring: AA, Aa, aA and aa gene pairs. In this case, the dominant trait would manifest itself in three out of four plants. This would explain the three-to-one ratio of dominant to recessive traits for all seven pairs of characteristics that Mendel observed in his plants’ second generation of offspring. In other words, it would explain why the recessive trait disappeared in the first generation of crosses (AA genes with aa genes) and yet manifested itself in one-fourth of the second-generation offspring.
Today, the idea of two separate hereditary factors that split during sexual reproduction is known as the principle of segregation. What Mendel called elements are now known as genes.
Rigorous Observation + Controlled Experimentation = Scientific Method
In addition to these critically important findings, Mendel is credited with applying the scientific method to biology. Prior to his time, most naturalists relied on their powers of observation and reasoning to explain the world around them. Mendel, on the other hand, used observation and reasoning to develop hypotheses that he then went on to test in repeatable, controlled experiments. He followed this up with mathematical analysis and strongly supported conclusions.
Nevertheless, Mendel’s contemporaries did not accept his conclusions. In fact, Mendel himself struggled with them. After he published the results of his experiment with pea plants in 1865, he attempted to confirm the results by studying hawkweed. Unfortunately, for him, the same patterns did not emerge. Today we know that this is due to the fact that hawkweed reproduces sexually and asexually, the latter of which follows a different set of rules than the former. Likewise, Mendel had little success studying hereditary elements in bees, in part because their reproduction was so hard to control.
In the end, Mendel’s genius was to isolate and examine specific traits individually, and to do so systematically with exacting precision over several generations of plants. Equally important, he counted all traits in every offspring, thereby generating a large amount of data, which he analyzed statistically.
In so doing, Mendel’s experiment not only revealed the mystery of inheritance, it also advanced other fields of science by helping to develop the rigorous scientific method of experimentation and analysis.