Pea Plants - Advanced
Douglas Wilkin, Ph.D.
Niamh Gray-Wilson
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Printed: February 25, 2017
AUTHORS
Douglas Wilkin, Ph.D.
Niamh Gray-Wilson
www.ck12.org
C HAPTER
Chapter 1. Pea Plants - Advanced
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Pea Plants - Advanced
Learning Objectives
• Explain why and how Mendel studied pea plants.
• Distinguish between characteristics and traits.
• Explain how Mendel was able to control pollination of the pea plants.
What’s so special about this plant?
The pea plant Pisum sativum has purple and white flowers. These flowered plants are not just pretty to look at. These
plants led Gregor Mendel to unlock the secrets of heredity, beginning the field of genetics. For his efforts, Mendel
is widely known as the Father of Genetics, even though he knew nothing of the genetic material, DNA. The laws he
developed apply to all sexually reproducing life, and are the basis for beginning to understand many human diseases.
Mendel and the Pea Plant
Prior to Mendel’s studies, it was commonly believed that offspring were a "mix" of their parents (the blending
theory of inheritance). For example, if a pea plant had one short parent and one tall parent, that pea plant would
be of medium height. It was believed that the offspring would then pass on heritable units, or heritable factors,
for medium sized offspring. (Today we know these heritable units are genes; however, Mendel did not know of
the concept of a gene or of DNA.) Mendel noted that plants in the monastery gardens sometimes gave rise to
plants that were not exactly like the parent plants, nor were they a “mix” of the parents. He also noted that certain
traits reappeared after “disappearing” in an earlier generation. Mendel was interested in finding out if there was
a predictable pattern to the inheritance of traits. Between 1856 and 1863 he grew and analyzed about 29,000 pea
plants in the monastery garden. It was Mendel’s knowledge and use of mathematics in his studies that allowed him
to analyze his results like no one before him.
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Mendel may have chosen to study peas and pea plants because they are fast-growing plants that are available in
different varieties. For example, one variety of pea plant has white flowers, as shown in Figure 1.1, while another
variety has purple flowers. But no variety has a pinkish-blend of the purple and white; there is no "middle" color of
flower in this particular species, P. sativum.
FIGURE 1.1
Pisum sativum, the pea plant species that Mendel studied.
Mendel chose to study seven characteristics of pea plants. A characteristic is a heritable feature, such as flower
color. Each characteristic Mendel chose to study occurred in two contrasting traits. A trait is a heritable variant of
a characteristic, such as purple or white flower color. Once again, no "blended" traits were observable. Figure 1.2
lists the seven characteristics Mendel studied and their two contrasting traits.
FIGURE 1.2
Mendel investigated seven different characteristics in pea plants.
In this chart,
cotyledons refer to the tiny leaves inside
seeds. Axial pods are located along the
stems. Terminal pods are located at the
ends of the stems.
Pea Plant Pollination
In order to study these characteristics, Mendel needed to control the pollination of the pea plants through artificial
fertilization. Pollination occurs when the pollen from the male reproductive part of a flower, called the anthers,
is transferred to the female reproductive part of a flower, called the stigma. Pea plants are self-pollinating, which
means the pollen from a flower on a single plant transfers to the stigma of the same flower or another flower on the
same plant. In order to avoid self-pollination, Mendel removed the anthers from the flowers on a plant. He then
carefully transferred pollen from the anthers of another plant and using a small paintbrush, dusted the pollen onto
the stigma of the flowers that lacked anthers. This process caused cross-pollination. Figure 1.3 shows the location
of the male and female parts of a flower. Cross-pollination occurs when pollen from one flower pollinates a flower
on a different plant. In this way, Mendel controlled the characteristics that were passed onto the offspring. The
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Chapter 1. Pea Plants - Advanced
product of cross-pollination is known as a hybrid. The ability to cross-pollinate pea plants allowed Mendel to study
the offspring of two parents with different traits, such as a tall stem or a short stem.
FIGURE 1.3
Summary
• Mendel used the pea plant in his studies for numerous reasons.
• Mendel’s use of mathematics in his pea plant studies was important to the confidence he had in his results.
• The ability to cross-pollinate the pea plants allowed Mendel to carefully control his studies.
Review
1.
2.
3.
4.
What is a characteristic? List the seven characteristics that Mendel investigated in pea plants.
How does pollination occur?
How did Mendel control pollination in pea plants?
What is a hybrid?
References
1. Flickr:net_efekt. http://www.flickr.com/photos/wheatfields/2670660145/ . CC BY 2.0
2. Jodi So and Rupali Raju. CK-12 Foundation . CC BY-NC 3.0
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