Lecture 3- Mendelian genetics

Lecture 3- Mendelian genetics - Mendelian Genetics:...

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Mendelian Genetics: Discrete Variation By discussing heredity as a function of how cells divide, we have ignored the historical order of discovery. Long before organisms were understood to be made up of cells, people tried to understand the patterns underlying inheritance. There is a fundamental and fascinating puzzle that underlies inheritance. The puzzle of heredity is that children resemble their parents but no children of the same parents are exactly alike (except identical twins). People have been aware of this puzzle for a very long time. In fact, I would argue that people pondered the riddle of heredity ever since they understood that animals (including humans) have a mother and a father. From the concept of a mother and a father, who combine some mysterious essence to produce offspring, two things immediately become clear even to young children: 1) for a given character (color, stature, facial features etc.) offspring share traits (light or dark color; short or tall stature) with their parents. Consider the character of skin color. For better or for worse, we are keenly aware of skin color. Therefore I imagine that you have observed that two Africans (who have the trait of dark skin) never have Caucasian children (who have the trait of light skin) and vice versa. These traits depend entirely on who ones parents are and are not significantly influenced by the environment. No amount of time spent in Africa will make a Caucasian an African (a good tan is the best they can hope for). However, if an African has children with an Caucasian, the children will share both African and Caucasian traits. For instance, they will have an intermediate skin color. However, two siblings from the same parents may not have the exact same skin color. When Gregor Mendel did his experiments in the 1860s, very little had been added to our understanding of heredity for thousands of years. Even Darwin, who based his whole theory of evolution (1859) on what he called "descent with modification", which is basically just a shorthand for "children resemble parents but siblings are not identical", had little to add. And not because he didn't try. Darwin spent a lot of time breeding fancy domesticated pidgeons to figure out a pattern to heredity. Mendel, Darwin's contemporary, succeeded where Darwin failed. Part of the problem was that many traits appear to show continuous variation. For example, skin color in people seems to consist of a continuous spectrum from black to white. Children of an African/Caucasian union are rarely black or white but rather intermediate, to one degree or another, between the two parents. This sort of blending of traits seemed to be the rule. At one time people believed that once blended, traits could not be separated again. Mendel's key insight was to concentrate on the exceptions to the rule i.e. those cases where
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This note was uploaded on 04/07/2010 for the course BIOL biol 112 taught by Professor Dent during the Winter '10 term at McGill.

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Lecture 3- Mendelian genetics - Mendelian Genetics:...

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