The Gregor Mendel Experiments

http://en.wikipedia.org/wiki/Gregor_Mendel

http://anthro.palomar.edu/mendel/mendel_1.htm

http://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593#

Ever wonder why you are the only one in your family with your grandfather's nose? The way in which traits are passed from one generation to the next-and sometimes skip generations-was first explained by Gregor Mendel. By experimenting with pea plant breeding, Mendel developed threeprinciples of inheritance that described the transmission of genetic traits, before anyone knew genes existed. Mendel's insight greatly expanded the understanding of genetic inheritance, and led to the development of new experimental methods.

Understanding Dominant Traits

Before Mendel's experiments, most people believed that traits in offspring resulted from a blending of the traits of each parent. However, when Mendel cross-pollinated one variety of purebred plant with another, these crosses would yield offspring that looked like either one of the parent plants, not a blend of the two. For example, when Mendel cross-fertilized plants with wrinkled seeds to those with smooth seeds, he did not get progeny with semi-wrinkly seeds. Instead, the progeny from this cross had only smooth seeds. In general, if the progeny of crosses between purebred plants looked like only one of the parents with regard to a specific trait, Mendel called the expressed parental trait the dominant trait. From this simple observation, Mendel proposed his first principle, the principle of uniformity; this principle states that all the progeny of a cross like this (where the parents differ by only one trait) will appear identical. Exceptions to the principle of uniformity include the phenomena of penetrance, expressivity, and sex-linkage, which were discovered after Mendel's time.

Mendel worked with seven characteristics of pea plants: plant height, pod shape and color, seed shape and color, and flower position and color. With seed color, he showed that when a yellow pea and a green pea were bred together their offspring plant was always yellow. However, in the next generation of plants, the green peas reappeared at a ratio of 1:3. To explain this phenomenon, Mendel coined the terms “recessive” and “dominant” in reference to certain traits. (In the preceding example, green peas are recessive and yellow peas are dominant.)

He came to three important conclusions from these experimental results:

1.	that the inheritance of each trait is determined by "units" or "factors" that are passed on to descendents unchanged      (these units are now called genes )
2.	that an individual inherits one such unit from each parent for each trait
3.	that a trait may not show up in an individual but can still be passed on to the next generation. The Experiment with Yellow and Green Seeds. The Experiment with Yellow and Green Seeds. The Experiment with two Traits. The Experiment with two Traits. The Seven Traits.