Mendel's trick: careful counting

Working over hundreds of years, plant and animal breeders produced the many domesticated species we know today.  However, their approach produced no deeper understanding of the mechanism of heredity. 

A scientific understanding of mechanism requires a quantitative approach. We need to count the specific traits observed in the offspring of controlled matings of well-defined organisms.   Based on these numbers, we can construct and test specific models of how heredity works.

This was the approach taken by the Austrian Gregor Mendel (1822-1884).  In 1856, Mendel began a series of experiments on plant breeding.  He established three criteria for deciding which plants would be the most suitable for his studies.

1. They should possess clearly distinguishable, discontinuous traits.
2. It must be possible to strictly control the breeding process.
3. Hybrids between the various stains must be fully fertile

Optional reading for the excessively enthusiastic: Mendel notes that his controlled breeding experiments could occasionally be disrupted by various natural events.  Look at Mendel's paper and answer the following questions.

1. What kinds of events did he specifically describe? How could these problems be avoided? (hint: read section 3).
2. Mendel used only "the most vigorous" plants in his experiments. Why?
3. How could the beetle Bruchus pisi disrupt Mendel's results?

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Mendel used varieties of garden pea Pisum sativum.   In this plant, the flowers contain both male and female organs - if left to their own devices there is strong possibility of self-fertilization (self-pollination).

In genetics, mating two organisms is referred to a "genetic cross" or "cross" for short.

Mendel was able to exploit the structure of the pea flower to generate crosses in which he knew which plant was the maternal parent and which was the parental parent (see applet below).

Mendel obtained 34 varieties of P. sativum from various seedmen and then spent two years determining whether these varieties 'bred true" for specific traits, i.e. whether self-fertilized plants always produced offspring similar to the parent.

After these initial studies, he chose 22 varieties to study further.

These varieties differed from one another in "length and color of the stem; in the size and form of the leaves; in the position, color, size of the flowers; in the length of the flower stalk; in the color, form, and size of the pods; in the form and size of the seeds; and in the color of the seed-coats and of the albumen (endosperm)" - G. Mendel

A particular plant was either tall or short, had white or purple flowers, etc.  Which traits an individual displayed could be determined unambiguously – they could be counted.

• Why did it matter to Mendel that a variety "breed true"?
• Why did it matter that traits were discontinuous?  Is it possible to analyze continuous traits?