Mendel and his peas

Do you have your father’s nose? Do people talk about how you “got those eyes from your mother”?

They probably don’t mean that you are suspected of sneaking up on your father while he took a nap, removing his nose, and Krazy Glueing it to your own face. They’re taking about the fact that we inherit some of our characteristics from our parents. The shape of our facial features, color of our eyes, and other visible characteristics are influenced by this business of inheritance, or heredity.

The same goes for other animals with whom we share Earth, and for plants. In fact, it was a common, ordinary plant — the garden pea — that got people thinking seriously about just how inheritance works.

Gregor Mendel, 1822-84

There had been plenty of scientific thought before the Austrian monk, Gregor Mendel, did his lengthy and detailed experiments with peas between 1843 and 1868. The Greek philosopher Aristotle (384-322 B.C.), knew that both male and female animals were required for reproduction, but then he proclaimed that the male provided the “active spirit” of any offspring, while the female served as little more than a holding place for the developing goat, puppy, or human baby. A male offspring was a sign that all had gone well; a female meant there had been a foul-up somewhere along the line. As silly as this theory sounds nowadays, it was accepted by many people for hundreds of years.

That theory was largely replaced in the 17th century by the idea of “preformation.” Future generations were already in place (“pre-formed”), folded up into microscopic packages, inside their parents. Then, in the 18th century, came the idea of “blending.” Both mother and father contributed equally to the offspring this time, but their contributions were blended: If a red flower bred with a white flower, the resulting seeds would produce pink flowers. Unfortunately for the blending enthusiasts, the theory didn’t work all the time.

Then came Mendel.
Then came Gregor Mendel (1822-84). Scientific discoveries are hardly ever the product of a single genius who mixes a few chemicals and yells “Aha!” and “Eureka.” But in this case, Mendel came quite close to fitting that stereotype. The monk used his training in mathematics in searching for universal rules of heredity. Much, if not most, scientific research has as its aim the discovery of general rules that can be widely applied.

Mendel planted ordinary peas in a garden at his monastery in Brno, which is in what's now the Czech Republic. He bred the seeds that were produced, and kept careful records on the characteristics of the offspring. It must have been helpful that Mendel lived the patient, unhurried life of a monk: In his experiments he grew an estimated 10,000 pea plants over a period of eight years. Mendel sorted out his peas according to seven easily observable traits, or characteristics. These were:

the shape of the seed (round or wrinkled)
the seed color (yellow or green)
the seed’s coating (gray or white)
the shape of the pod (puffy-looking or wrinkled)
the pod color (green or yellow)
the pattern by which the flowers were distributed along the stem (axial, or growing out of different points along the stem, or terminal, growing out of the end only)
the stem length (long or short).

It doesn’t take an electronic calculator (which Mendel didn’t have and hadn’t been invented, anyway) to see that many different possible combinations of traits were possible.

Peas were Mendel’s plants of choice because their reproduction is easy to control. Peas have both male reproductive organs (anthers) and female ones (stigmas and ovaries) on the same plants. A scientist, farmer, or curious student who wants to breed plants with different traits (“cross-pollinate” them) can clip off the anthers of Plant A, collect the dust-like pollen from the anthers of Plant B, place that pollen on the female parts of Plant A (a small paint brush is an excellent pollen-mover), and let Nature take its course. The resulting plant will contain characteristics of both Plants A and B. We now refer to these characteristics as genes.

One of Mendel’s first discoveries was that the characteristics did not “blend,” as prevailing scientific opinion had it. That is, if he crossed a long-stemmed plant with a short-stemmed one, the offspring were not all medium-stemmed. Instead, he found that in pairings of two traits, one always turned out to be dominant and the other recessive. How’d he know which was dominant? All the plants in the first generation of a pea-crossing possessed one characteristic but not the other. That characteristic was the dominant one.

There's always a “but.”
But (and there almost always is a “but” in science): If he crossed members of that first generation (which plant breeders refer to as “F1 hybrids”) with one another, the second generation (known as “F2 hybrids”) contained some plants with the recessive characteristic as well as those with dominant ones.

For example: Cross a round, smooth-skinned pea with a wrinkled one. If you get four F1 hybrids, all will be rounded. Roundness is the dominant characteristic. Produce a second generation (F2), and on average you’ll get three plants with rounded peas and one with wrinkled peas. The recessive gene for wrinkleness, hidden in the first generation, now appears.

Mendel could see, then, that the recessive gene had not been discarded by the breeding process. Rather, it hung around in the genetic background, coming to the surface (“expressing” itself, as scientists put it) in later generations. Mendel continued his experiments and note-taking, doing more and more complicated crossings.

In the process, he produced two important scientific rules (called “laws”) about heredity that are still valid today. One is the “law of segregation,” and the other is the “law of independent assortment.”

So Mendel’s patient discoveries, which he published in 1866, shook the world of science, right?

No. They were widely ignored until 1900, when they were rediscovered by three researchers, working independently.

Why wasn’t such earth-shaking news properly appreciated? One possible reason is that Mendel’s findings knocked the stuffing out of the popular theory of blending, and a lot of scientists and non-scientists weren’t ready to accept that in 1866. Another is that Mendel was a whiz at mathematics, and used math extensively in reaching his conclusions. His fellow biologists had not yet accepted mathematics as an important tool. Today, the study of biology could hardly live without math.

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