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Introduction to Mendelian Genetics

Author(s): Lisa M. Meffert, PhD

Blending Model of Inheritance vs. Particulate Model of Inheritance

Mendel used experimental approaches to characterize a particulate model of inheritance. In doing so, he developed the three Laws of Inheritance: the Law of Segregation, the Law of Independent Assortment, and the Law of Dominance.

Before Mendel's experiments, prevailing thought about heritable characteristics revolved around the idea that contributions from each parent somehow blended together (the blending model) to generate inheritance patterns in offspring. For example, a tall father and a short mother would produce children of intermediate height. (Note that human height actually is determined by many genetic and environmental factors.) This blend then would be passed on to the next generation. The problem with this model was that it allowed for no variation over time. In our example, eventually everyone would be average or uniform in height. Clearly, this is not what really happens.

According to Mendel's particulate model, an individual receives some form of a particle from each parent. We now call this particle the "allele." Alleles represent the many forms that a gene can take. The two particles then dictate the inheritance pattern in the offspring. Returning to the example we used above, a tall father and a short mother still could produce offspring of average height. However, in the particulate model, the contributions from the parents (particles) are separated from one another when being passed on to the next generation. In this way, particles can continue to combine in various ways, thus maintaining variation through time.

In this module, we will examine how Mendel made this important discovery.