Mendel: The Father of Genetics

Mendel grew up on a small farm in Austria, so he got an intuitive understanding of plant and animal breeding. Mendel wasn’t an ordinary monk. In the 1850’s, his order sent him to study at the University of Vienna, one of the leading universities of Europe. He was taught by many outstanding scientists there, most notably the physicist Christian Doppler. When he returned to the monastery, he began to study heredity in garden peas.

At the time of Mendel’s work, chromosomes hadn’t been described, let alone mitosis and meiosis. Nothing was known about the physical basis of inheritance. It was obvious, though, as it had been for many thousands of years, that offspring tend to resemble their parents. It also was obvious that different combinations of traits in parents would appear to be mixed in different ways in their offspring, sometimes surprisingly so.

At that time, the dominant theory about how inheritance worked suggested that some material from the two parents was blended together when offspring were produced. You can think of this like mixing paint. What scientists thought was that when offspring were produced, paint from the two parental buckets would simply be poured together into the offspring’s bucket, and the result would be some mixture of the two. If for example an animal with light colored fur mated with a dark colored one, the result would be an intermediate color. Indeed, sometimes that is observed.

The blending hypothesis didn’t account for many other observations made by plant and animal breeders, though. Specifically, the blending hypothesis predicts that over time, all variation within a species should vanish. Think of blending buckets of paint. If we start with a whole bunch of colors in different buckets, and as those buckets are mixed together, over time we’ll just have buckets with the same color.

In the real world, however, plant and animal breeders realized that even when individuals in a population mated randomly for a very long period of time, there always remained variation among individuals. Also, sometimes traits would appear in offspring that weren’t present in either of the parents.

These were the problems Mendel was trying to address when he began his work on pea plants. To understand Mendel’s work, let’s see the very first experiment he did: the monohybrid cross.

Mendel’s most famous experiment involved looking at the inheritance of flower colors. In these pea plants, flower colors occur in only one of two forms: purple or white. We call this kind of character a “dichotomous character”. It’s one thing or the other. Pea plants have many such dichotomous characters, not only flower color, but also seed color, the shape of the seed and the height of the plant.

Mendel understood that the existence of this kind of dichotomous traits couldn’t be reconciled easily with the blending hypothesis, so he decided to investigate how these traits were transmitted across generations.

Mendel began his first experiment with parents that came from what we call true breeding lines. These were pea plants that would produce only one form of the trait. For example, one plant that produced only white flowers, or only purple flowers. He then crossed a white flower plant with a purple flower plant. This is called a monohybrid cross because he is hybridizing two varieties that differ in only a single character.

When Mendel performed this cross, he found a very surprising result. Remember, one parent was purple and the other white. When he crossed them, all of the offspring were purple.

We started with a parental generation, one purple and one white. The offspring of this parental generation are called the F1 generation. The result that Mendel got was surprising because it completely contradicted the blending hypothesis. The blending hypothesis predicted that he’ll get an intermediate color, like a light purple. But no, all of the offspring were purple flowered. The white flower color seemed to be completely lost.

Even more surprising was what happened when he crossed these F1 individuals. What he found in the so called F2 generation was that the white flower color reemerged. He would have some purple flower individuals and some with white flower individuals. Furthermore, these individuals would always occur in approximately the same ratio. There always would be about 3 purple individuals for every one white individual.

Mendel did this kind of cross with a number of other dichotomous traits, like seed color and seed shape, and he always got the same result. One trait would disappear in the F1. Then it would reappear in the F2 in the ratio of 3 to 1. What could account for this pattern of inheritance? How could you have one trait lost completely and then have it again?

In trying to solve this puzzle, Mendel developed a hypothesis that explains what genes do on chromosomes when they are transmitted, even though he had no idea what genes or chromosomes were. In so doing, he essentially established modern genetics. In the next article we’ll see the hypothesis Mendel developed to explain the surprising and exciting results he got.

Pea Images Source: Wikimedia Commons. Here and here.

1 Comment:

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