3.11: Genetics
- Page ID
- 62211
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The father of genetics is actually a monk named Gregor Mendel. Remember that Gregor Mendel (1822-1884) didn't know about DNA when he did his experiments, he didn't see meiosis in the microscope, he wasn't directly involved in the debates over evolution, but he found one of the sources of variation that Darwin's theory of natural selection relies on, and he discovered two important principles that are the foundation of genetics: The Principle of Segregation and The Principle of Independent Assortment. Darwin knew that variation was crucial to his theory, but he didn't know the source of variation.
The pea plant has variation. Some seeds are smooth, some wrinkled; some yellow, some green. Some pods are inflated, some constricted; some green, some yellow. Some flowers are purple, some white; some along the stem, some at the top. Some stems are tall, some are short. Mendel was careful to exclude other kinds of variation: how some plants are eaten by snails, some don't get enough water, some too much sun, some are cooked in soup, some peas are overcooked, some shot through straws. Mendel ignored all these things that happen to peas and only paid attention to this first set of variations, the either/or inherent characteristics that can be seen.
Mendel was rediscovered around 1900. Theories of inheritance at the time of Mendel focused on blending, for example, one parent with extremely dark skin and one parent with extremely light skin have a child who is neither very light, nor very dark, but a color that is in between the extremes. But when Mendel bred purple flowers with white flowers, he got only purple flowers, and then when he bred those purple flowers together, in the next generation he got mostly purple but some white ones. The white flower trait disappeared and then came back. The purple color dominated the white one, but the recessive white color was not gone forever, it came back in a later generation. If you cross a purple flower with a white flower, Darwin would have expected a whitish-purple flower. What happened to the blending?
Mendel answered this with his Principle of Segregation. Mendel showed that each trait (seed color, seed shape, pod shape, pod color, flower color, flower position, stem length) is determined by a pair of characters, and they get them from their parents, one from the pollen cell and one from the egg cell, which come together to form the embryo. When the pollen and egg cells are made, these two characters are "segregated" so each egg and pollen cell has only one character. In genetics we now call these traits, genes, and the pair of characters is called a pair of alleles. From cellular biology, we now know that the segregation of alleles during the production of eggs and sperm is called meiosis.