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1.4: Genes are the Basic Units of Inheritance

1.4.1 Blending vs Particulate inheritance

The once prevalent (but now discredited) concept of blending inheritance proposed that some undefined essence, in its entirety, contained all of the heritable information for an individual. It was thought that mating combined the essences from each parent, much like the mixing of two colors of paint. Once blended together, the individual characteristics of the parents could not be separated again. However, Gregor Mendel (Fig 1.10) was one of the first to take a quantitative, scientific approach to the study of heredity.  He started with well-characterized strains, repeated his experiments many times, and kept careful records of his observations.  Working with peas, Mendel showed that white-flowered plants could be produced by crossing two purple-flowered plants, but only if the purple-flowered plants themselves had at least one white-flowered parent (Fig 1.11).  This was evidence that the genetic factor that produced white-flowers had not blended irreversibly with the factor for purple-flowers.  Mendel’s observations disprove blending inheritance and favor an alternative concept, called particulate inheritance, in which heredity is the product of discrete factors that control independent traits.

Figure 1.10:
Gregor Mendel. (Original-unknown-PD)

Figure 1.11:

Inheritance of flower color in peas. Mendel observed that a cross between pure breeding, white and purple peas (generation P) produced only progeny (generation F1) with purple flowers.  However, white flowered plant reappeared among the F2 generation progeny of a mating between two F1 plants.  The symbols P, F1 and F2 are abbreviations for parental, first filial, and second filial generations, respectively.

1.4.2 Genes and alleles

Mendel’s discrete “factors of heredity” later became known as genes. Each hereditary factor could exist in one or more different versions or forms, which we now call alleles. In its narrowest definition, a gene is an abstract concept: a unit of inheritance.  The connection between genes and substances like DNA and chromosomes was established largely through the experiments described in the remainder of this chapter.  However, it is worth noting that Mendel and many researchers who followed him were able to provide great insights into biology, simply by observing the inheritance of specific traits – genetics.