Two principles of heredity were formulated by Gregor Mendel in 1866, based on
his observations of the characteristics of pea plants from one generation to the
next. The principles were somewhat modified by subsequent genetic research.
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Mendel's Law of segregation: The characteristics of the
offspring are derived from both maternal and paternal factors. Every individual
has a pair of genes governing a particular characteristic (e.g. the color of the
eyes). During the formation of sex cells each pair is separated (segregated) so
that each sex cell (egg or sperm) carries only one form of each gene. The
offspring thus receives one from each parent and this pair of genes determines
how the caracteristic is expressed (e.g. whether the child's eyes are blue or
brown).
Mendel's law of independent assortment: When considering more
than one gene, Mendel noted that two characteristics do not always appear
together. For instance a mother with blonde hair and blue eyes may have a
blonde-haired child with brown eyes. Thus differnt characteristics can be
independently inherited.
