Mendel’s First Experiment Advanced
Douglas Wilkin, Ph.D.
Niamh Gray-Wilson
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Printed: November 30, 2016
AUTHORS
Douglas Wilkin, Ph.D.
Niamh Gray-Wilson
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C HAPTER
Chapter 1. Mendel’s First Experiment - Advanced
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Mendel’s First Experiment Advanced
Learning Objectives
• Identify the terms used to describe the three generations in Mendel’s studies.
• State one reason for carrying out a monohybrid cross.
• Identify the traits that appeared in Mendel’s F2 generation.
Peas. Some round and some wrinkled.
That’s what Mendel asked. He noticed peas were always round or wrinkled, but never anything else. Why? Seed
shape was one of the traits Mendel studied in his first set of experiments. Through his analysis of these peas,
analyzing seven characteristics, Mendel was able to develop two Laws of Inheritance. And he did this essentially
through just two types of experiments.
Mendel’s First Experiment
Mendel began his studies by growing plants that were true-breeding for a particular trait. A true-breeding plant will
always produce offspring with that trait when they self-pollinate. For example, a true-breeding plant with yellow
seeds will always have offspring that have yellow seeds. To obtain these plants, Mendel allowed plants with just one
trait, such as purple flowers or white flowers, to self-pollinate for many generations. He allowed this to continue
until he was sure the offspring from those plants always had only just the one trait, in this case, either purple flowers
or white flowers.
In his first experiment, Mendel cross-pollinated two true-breeding plants of contrasting traits, such as purple and
white flowered plants. The true-breeding parent plants are referred to as the P generation (parental generation). The
hybrid offspring of the P generation are called the F1 generation (first filial generation). The hybrid offspring of the
F1 generation are called the F2 generation (second filial generation).
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Monohybrid Crosses
Mendel first worked with plants that differed in a single characteristic, such as flower color. A hybridization is a
cross between two individuals that have different traits. A hybridization in which only one characteristic is examined
is called a monohybrid cross. The offspring of such a cross are called monohybrids. Mendel noted that hybridizing
true-breeding (P generation) plants gave rise to an F1 generation that showed only one trait of a characteristic. For
example, a true-breeding purple-flowering plant crossed with a true-breeding white-flowering plant always gave rise
to purple-flowered hybrid plants. There were no white-flowered hybrids. Mendel wanted to know what happened to
the white-flowers. If indeed a "heritable factor" for white-flower had disappeared, all future offspring of the hybrids
would be purple-flowered - none would be white. To test this idea, Mendel let the F1 generation plants self-pollinate
and then planted the resulting seeds.
FIGURE 1.1
This diagram shows Mendel’s first experiment with pea plants. Mendel started by
crossing a true-breeding purple-flowering
plant with a true-breeding white-flowering
plant. The F1 generation contained all
purple flowers. The F2 generation results
from self-pollination of F1 plants, and contained 75% purple flowers and 25% white
flowers.
Mendel’s Results
The F2 generation plants that grew included white-flowered plants. Mendel noted the ratio of white flowered plants
to purple-flowered plants was about 3:1. That is, for every three purple-flowered plants, there was one white flowered
plant. Figure 1.2 shows Mendel’s results for the characteristic of flower color.
Mendel carried out identical studies over three generations, (P, F1 , and F2 ), for the other six characteristics and found
in each case that one trait “disappeared” in the F1 generation, only to reappear in the F2 generation. Mendel studied
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Chapter 1. Mendel’s First Experiment - Advanced
a large number of plants, as shown in Table 1.1. His use of statistics to demonstrate the repeated 3:1 ration of traits.
Because of the repeatable nature of his findings, Mendel was confident that the ratios of different traits in the F2
generation were representative. As shown in the table, Mendel called the trait that appeared in the F2 75% of the
time the dominant trait, and the trait that reappeared in the F2 the recessive trait.
FIGURE 1.2
TABLE 1.1: Results of F1 Generation Crosses for Seven Characteristics in
Characteristic
Dominant Trait
Recessive Trait
Ratio
white
terminal
F2
Generation
Dominant:Recessive
705:224
651:207
Flower Color
Flower Position on
stem
Stem Length
Pod Shape
Pod Color
Seed Shape
Seed Color
purple
axial
tall
inflated
green
round
yellow
short
constricted
yellow
wrinkled or angular
green
787:277
882:299
428:152
5474:1850
6022:2001
2.84:1
2.95:1
2.82:1
2.96:1
3.01:1
3.15:1
3.14:1
MEDIA
Click image to the left or use the URL below.
URL: https://www.ck12.org/flx/render/embeddedobject/155432
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Summary
• Mendel’s use of mathematics in his pea plant studies was important to the confidence he had in his results.
• Mendel carried out his first experiments with true-breeding plants and continued them over a span of three
generations.
• For each of the seven characteristics Mendel studied, he observed a similar ratio in the inheritance of dominant
to recessive traits (3:1) in the F2 generation.
Review
1.
2.
3.
4.
5.
Why was Mendel’s understanding of mathematics and science important for his research?
Describe in specific terms Mendel’s first set of experiments.
What is a true-breeding plant?
How did the appearance of Mendel’s F1 generation differ from the appearance of the P generation?
Assume you are investigating the inheritance of stem length in pea plants. You cross-pollinate a short-stemmed
plant with a long-stemmed plant. All of the offspring have long stems. Then, you let the offspring selfpollinate. Describe the stem lengths you would expect to find in the second generation of offspring.
References
1. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. CK-12 Foundation . CC BY-NC 3.0
2. CK-12 Foundation, using purple P. satvium image by Forest and Kim Starr and white P. satvium image by
Flickr:net_efekt. Purple P. satvium: http://www.flickr.com/photos/starr-environmental/9196383877/; White
P. satvium: http://www.flickr.com/photos/wheatfields/2670660145/ . CC BY-NC 3.0 (both flower images
available under CC BY 2.0)
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