key concept 12.1 Inheritance of Genes Follows Mendelian Laws

241

Genetics, the field of biology concerned with inheritance, has a long history. There is good evidence that people were deliberately breeding animals (horses) and plants (the date palm tree) for desirable characteristics as long as 5,000 years ago. The general idea was to examine the natural variation among the individuals of a species and “breed the best to the best and hope for the best.” This was a hit-or-miss method—sometimes the resulting offspring had all the good characteristics of the parents, but often they did not.

focus your learning

  • Two hypotheses have been proposed to explain how characteristics are passed from one generation to the next.

  • Mendel’s crosses of pea plants led to the formulation of laws of genetic inheritance.

  • Mendel’s laws of inheritance apply to all sexually reproducing organisms, including humans.

By the mid-nineteenth century, two theories had emerged to explain the results of breeding experiments:

  1. The hypothesis of blending inheritance proposed that gametes (sex cells such as eggs or sperm) contained hereditary determinants (what we now call genes) that blended when the gametes fused during fertilization. Like inks of different colors, the two different determinants lost their individuality after blending and could never be separated. For example, if a plant that made smooth, round seeds was crossed (bred) with a plant that made wrinkled seeds, the offspring would be intermediate between the two and the determinants for the two parental characteristics would be lost.

  2. The hypothesis of particulate inheritance proposed that each determinant had a physically distinct nature; when gametes fused in fertilization, the determinants remained intact. According to this hypothesis, if a plant that made round seeds was crossed with a plant that made wrinkled seeds, the offspring (no matter the shape of their seeds) would still contain the determinants for the two characteristics.

The story of how these competing hypotheses were tested provides a striking example of how the scientific method can be used to support one theory and reject another. In the following sections we will look in detail at experiments performed in the 1860s by an Austrian monk and scientist, Gregor Mendel, whose work clearly supported the particulate hypothesis.