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Natural Science - Year II

Unit 45: Gregor Mendel and Patterns of Inheritance

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History Weblecture for Unit 45

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History Lecture for Unit 45: Mendel and Genetic Inheritance

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Genetic Inheritance

One serious problem for the theories of natural selection and evolutionary descent was that Darwin could not explain how traits were inherited by each generation, since he did not have a theory of genetics, or how new characteristics developed in an old species, since he rejected the Lamarckian idea of acquired traits. It was obvious that children inherit traits from their parents -- not only things common to all humans, like eyes and hair and fingers, but more specific forms of these things, like eye color, hair color, and the shape and even number of fingers.

Darwin apparently knew nothing about the work of his contemporary, Gregor Mendel, had done on the inheritance of traits. Mendel was an Augustinian monk who became interested in cross-breeding plants. Over a decade-long experiment, he carefully cross-bread species of peas to see how they inherited specific traits.

Gregor Mendel's Experiments

Read Gregor Mendel's brief biography.

  • What is an inherited trait?
  • How long did Mendel study his pea plants?
  • Many people think that Mendel's experiment, like Galileo's experiments with the rolling balls on the inclined plane, were well-designed. Would you agree? What makes a good experiment? Why was Mendel's approach to the problem a good one?
  • When was Mendel's paper published? Where?
  • Why wasn't Mendel's theory widely accepted in his lifetime?

Mendel's laws of inheritance
Law of Segregation The alleles for each gene segregate from one another so that each gamete carries only one allele for each gene.
Law of Independent Assorment Genes for different traits can segregate independently during the formaiton of gametes.
Law of Dominance Some alleles are dominant and some are recessive; an organism with at least one domininant allele will display the effect of the dominant allele.

Mendel's experiments and statistical natural laws

Mendel's experiments are often considered excellent examples of the scientific method. He had a good working hypothesis: he thought that inheritance of simple traits (where there are only two possible variations, one dominant and one recessive) could be predicted using statistical methods. He set up his experiments carefully, controlling pollination to ensure that his plants "bred true" and had only the inherited factors he wanted for a given trial. He realized that recessive traits could remain hidden through multiple generations, so he chose fast-breeding and developing plants and carried out his experiments over several years, with meticulous notes on each generation. Ultimately he observed tens of thousands of plants, giving him a big enough population to test his statistical predictions. We now call his rules of inheritance "laws".

It's important to realize here that many modern scientific laws are based on statistical predictions, rather than absolute behaviors. Consider the "law of gravity", Newton's principle that the force of gravity FG between two objects is dependent on the two masses M and M involved and the distance r between them: FG = GMm/r2. This law is absolute: it applies to any two individual masses M and m, regardless of whether the masses are subatomic particles or stars, and whether the distance is fractions of a nanometer or parsecs. This absolute law allows us to send spacecraft to Mars and know that if we calculated the trajectories correctly, the gravitational fields of the sun and planets will put the spacecraft at the position of Mars at exactly the right time to intercept the moving planet.

In a statistical law, we look at the behavior of huge numbers of similar objects, and predict either the overall behavior of the system as a whole. We may be able to determine the likelihood of one or more behaviors of an individual, but we cannot predict the exact behavior of any given individual. Mendel's laws allowed him to predict the likelihood that a particular plant would have green color or a wrinkled appearance, but he could not guarantee the plant would have one or another, since other outcomes were also possible, although less likely.

We need to understand this difference, because we will see it again in discussions of theromodynamics and the laws of entropy, as well as quantum mechanics and the uncertainty principle. In both these areas, we can predict the actual behavior of large groups of objects, but not the individual behavior of any one object. If we apply statistical laws as though they were absolute laws to individual objects, we run the risk of being wrong in a given instance, with possibly disastrous results.

Acceptance of a Theory

In the last few units, we've seen how lack of evidence kept one theory, Alfred Wegener's proposal of continental drift, from being accepted, and how cultural and religious convictions challenged the theories of Charles Darwin, and created problems with it beign widely accepted, even today. Gregor Mendel's theory also experienced a long period between proposal and acceptance, largely because of the way it was published: in German, in a journal that was not widely read.

As we discovered at the outset of Natural Science I, writing is important! Publication of scientific ideas is the means by which people learn and can review and accept or reject theories. The exchange of papers by the Manchester Society allowed Dalton to publish his ideas and get responses and confirmation of his theories, even before the formal publication of his books. The lack of publication in a widely circulated venue kept Mendel's theories in obscurity for three decades.

Over the twentieth century, a very strict process of publication was followed: a scientist would draft a paper and send it to a journal, the editor would submit it to other experts for review, and the paper would be published or rejected by the journal. In some cases, the scientist might submit the article to another journal, but frequently the refusal to print a particular article would condemn the article (and often its author) to obscurity. The process was intended to prevent sloppy work from reaching and distracting serious scientists, and to make sure that credit was given where it was really deserved for actual discoveries — an important consideration when jobs, grants for research, or even the money and fame of the Nobel Prize could be the reward for discoveries.

One of the drawbacks, of course, is that a theory that challenges established ideas of either the editor or the group of reviewers could be suppressed for personal or political, rather than valid scientific reasons. To some extent, the availability of publising ideas on the Web is addressing the problem, allowing an author to put his ideas before the scientific community and the public when the traditional journals have rejected them. This situation, though, makes it even more imperative that the general public be well-educated in scientifc matters, since they must just Website content on their own, and separate the valid reports from well-intentioned but sloppy work as well as prejudiced or partial reports that are meant to mislead the reader.

Study/Discussion Questions:

Further Study/On Your Own