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The Principle of Segregation

Mendel concluded that each plant contained two “factors” for each trait that could be inherited by the progeny.  Each parent contributed one factor to the offspring. If the progeny received at least one factor that was dominant, the offspring would show that trait. Only offspring inheriting two recessive factors would show that characteristic. Since the two factors remain distinct and do not blend, Mendel called this the Principle of Segregation.


Mendel did not know what these factors were, but today we can explain his observations on a molecular level. Mendel’s “factors” are genes, and the different varieties of the genes (purple vs. white flowers, for example) are called alleles.  Mendel’s observations, particularly his Principle of Segregation, are easily explained with our knowledge of meiosis (see Chapter 18). During meiosis, the pairs of homologous chromosomes (those that carry the same information, or the same genes) line up and one chromosome of the pair moves to one pole of the cell while the other moves to the opposite pole. Therefore, one allele will be distributed to each new cell.

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