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2.1 The Puzzle of Inheritance 15

Figure 2.1 A family portrait. The extended Figure 2.2 Gregor Figure 2.3 Like begets like and unlike.
family shown here includes members of four Mendel. Photographed A Labrador retriever with her litter of pups.
generations. around 1862 holding one of © Saudjie Cross Siino/Weathertop Labradors
© Bruce Ayres/Getty Images his experimental plants.
© Science Source

doing, he inferred genetic laws that allowed him to make verifiable predictions about
which traits would appear, disappear, and then reappear, and in which generations.
Mendel’s laws are based on the hypothesis that observable traits are determined
by independent units of inheritance not visible to the naked eye. We now call these
units genes. The concept of the gene continues to change as research deepens and
refines our understanding. Today, a gene is recognized as a region of DNA that
encodes a specific protein or a particular type of RNA. In the beginning, however,
it was an abstraction—an imagined particle with no physical features, the function
of which was to control a visible trait by an unknown mechanism.
We begin our study of genetics with a detailed look at what Mendel’s laws are
and how they were discovered. In subsequent chapters, we discuss logical extensions
to these laws and describe how Mendel’s successors grounded the abstract concept
of hereditary units (genes) in an actual biological molecule (DNA).
Four general themes emerge from our detailed discussion of Mendel’s work. The
first is that variation, as expressed in alternative forms of a trait, is widespread in nature.
This genetic diversity provides the raw material for the continuously evolving variety of
life we see around us. Second, observable variation is essential for following genes from
one generation to the next. Third, variation is not distributed solely by chance; rather, it
is inherited according to genetic laws that explain why like begets both like and unlike.
Dogs beget other dogs—but hundreds of breeds of dogs are known. Even within a breed,
such as Labrador retrievers, genetic variation exists: Two black dogs could have a litter
of black, chocolate (brown), and yellow puppies (Fig. 2.3). Mendel’s insights help explain
why this is so. Fourth, the laws Mendel discovered about heredity apply equally well to
all sexually reproducing organisms, from protozoans to peas to dogs to people.

2.1 The Puzzle of Inheritance 3. Explain the importance of Mendel’s inclusion of
reciprocal crosses within his controlled breeding
program of pea plants.
4. Predict the type of progeny produced by Mendel’s
learning objectives crosses between pure-breeding plants with discrete,
antagonistic traits, such as purple versus white flowers.
1. Relate how Mendel’s experimental approach
is similar to the process of modern scientific
inquiry. Several steps lead to an understanding of genetic phenom-
2. Describe how Mendel cross-fertilized and self-fertilized ena: the careful observation over time of groups of organ-
pea plants. isms, such as human families, herds of cattle, or fields of