Saturday, September 24, 2011

Mendelian Genetics Part I

Gregor Mendel was an accomplished scientist, though he never would know this during his time. His studies of pea plants (as well as other organisms) laid the groundwork for the genetic breakthroughs that would come after his death. Mendel was a Augustinian monk that had ample time to experiment with his pea plants. The principles he was able to extract from his studies are the basics taught in high schools around the world.

 Mendel studied peas, but not just any peas. His peas were true-breeding, meaning that they self-fertilized and produced essentially clones of themselves.  Over the years of study, he bred plants with specific characteristics and then crossed different varieties of pea plants to test what their offspring would look like.

Importantly, Mendel considered discontinuous, contrasting traits: he only considered the traits that he could observe (such as color), came in a limited number of forms (green or yellow) and were easily distinguishable.  For these monohybrid crosses, Mendel considered seven different traits: seed shape (round versus wrinkled), seed color (yellow versus green), pod shape (full versus constricted), pod color (yellow versus green), flower color (violet versus white), flower position (axial versus terminal), and stem height (tall versus short).  Also important to Mendel's work was his use of mathematics and statistics to estimate the probabilities of a certain type of plant emerging from a certain type of pea plant cross.

The monohybrid cross is the consideration of one particular trait at a time.  For example, if looking at pea pod color, one might cross a yellow and a green pea plant and then determine how many of the offspring had yellow or green pea pods.  This type of cross is illustrated to the right in what's called a Punnett Square.  A Punnett square is a way of organizing the different traits expressed by the individuals being crossed.  The capitalized letters are the dominant traits, while the lower-cased letters are recessive.  Vertical columns traditionally represent females; horizontal, males.  Using the Punnett square, we can look at all the possible offspring that can emerge from a cross, which can then be used to determine probabilities associated with the offspring's traits.

From these monohybrid crosses, Mendel was able to make a few conclusions.  When he crossed two pea plants of distinct traits (the P1 generation, true-breeding variety) to produce progeny (the F1 generation) and then used the progeny to generate more progeny (the F2 generation), he found that the two parental traits were still present and unchanged.  This led to the hypothesis that each parent contributed equally to the inheritance of the "genetic determinants," which would have more technical and molecular definitions in the distant future.  These determinants were separated and segregated randomly to make gametes and produce the next generation of pea plants.

Mendel's principles from the monohybrid crosses can be summarized as follows:

  1. Hereditary determinants (unit factors) control traits that are in pairs in an individual.
  2. When two dissimilar unit factors for a trait are combined in one organism, one factor is dominant to the other recessive factor.
  3. When gametes are formed, the pair of unit factors separate and are equally likely to be separated into a gamete: the principle of segregation
In the next post, we'll examine dihybrid crosses and some more of the interesting findings from, of all things, pea plants.

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