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THE WORK OF GREGOR MENDEL
1. Introduction
Heredity is the passing of physical characteristics from parents to
offspring. The first experiments on heredity where done by a monk named Gregor
Mendel, who lived in Austria during the 1800s. Mendel was curious about the different
forms of physical characteristics, or traits, of bees and garden plants. Mendel’s work
on the common pea plant became the foundation of genetics, or the scientific study of
heredity.
2. What Did Actually Mendel Do?
Mendel investigated 7 pea traits: plant height, flower color, flower position on the stem, seed
shape, seed color, pod shape and pod color.
Mendel’s experiments involved breeding pea plants together. Normally, pea plants are selfpollinating, meaning that pollen from one flower can pollinate the eggs of the same flower. This results
in offspring that are identical to their parents (clones). They are also called purebred plants. A
purebred has the same version of one trait from both parents.
Mendel perfected a method that allowed him to cross-pollinate, or “cross” different pea plants
together. He would transfer the pollen from one plant’s flower onto another plant’s flower by hand,
creating hybrid plants. A hybrid is an organism that has two different versions of the same trait. In
other words, they are a mix of 2 different traits.
3. An Example of Mendel’s Work
In one experiment, Mendel crossed 2 parent pea plants with different heights. The parents were
called the “P generation”. He crossed purebred tall plants (T) with purebred dwarf plants (t) and
examined the first generation, called the “F1 generation”:
1. In the F1 generation, the tall trait appeared in 100% of the plants. The dwarf trait seemed to
completely disappear!
2. Then, Mendel let the F1 plants self-pollinate to create the second generation (the F2 generation). In
these plants, the dwarf trait reappeared! Even more surprising, 3/4 of the F2 plants were tall,
and 1/4 were dwarf. There was a clear 3:1 ratio!
4. An Explanation
A physical feature of a plant, such as height, is called a trait. Each plant must have two
“factors” for each trait, one from each parent. Some traits, like tall height, show up even if there is only1
factor present. These traits are called dominant traits because they dominate over the other traits.
They are indicated by a capital letter. Other traits can be hidden or covered up by the dominant trait.
These traits are called recessive traits. They are indicated by lowercase letters. Recessive traits only
show up if both recessive “factors” are present.
For example, a plant with one tall-height factor (T) and one dwarf-height factor (t) will be always
be tall, since the tall-height factor covers up the dwarf-height factor. The only way that dwarf height will
show up is if the plant possesses two dwarf-height factors (tt).
Below is a labeled diagram to explain the results of Mendel’s experiments on pea plant height:
Factors:
TT
tt
(purebreds)
Tt
Tt
TT
Tt
Tt
tt
(hybrids)
3
1
5. Mendel’s Final Conclusions
Mendel’s performed 1000s of other experiments, with most of them resulting in the same
patterns. Mendel proposed a hypothesis that would explain the patterns he observed:
1. Each plant must have two “factors” for each trait, one from each parent.
2. Some traits, like tall height, are dominant and will always show up if they are present.
3. Other traits, such as dwarf height, are recessive and can be masked by dominant traits.
Mendel’s conclusions formed the basis for modern day genetics. However, the terms have
changed. What Mendel called “traits” are now called genes and the different versions of each trait that
Mendel called “factors” are now called alleles.
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REVIEW - THE WORK OF GREGOR MENDEL
1. Match the term with its definition
1. heredity
a. The scientific study of heredity
2. genetics
b. physical characteristics in an organism
3. traits
c. the passing of traits from parent to offspring
2. What are “purebred” organisms? What are “hybrid” organisms?
3. Complete the flowchart below, which summarizes Mendel’s first experiment with pea plants.
Purebred tall plants are crossed with purebred _______________ plants.
F1 offspring are all _______________. F1 offspring are allowed to self-pollinate.
F2 offspring are __________ and __________. They are in a _____ to _____ ratio.
4. Circle the letter of each sentence that is true about traits.
a.
b.
c.
d.
Recessive traits are never present when dominant traits are present.
Traits are different forms of a characteristic.
Dominant traits always show up in the organism if even one trait is present.
Recessive traits hide dominant traits.
5. True or false? Only pea plants that have two recessive traits for dwarf height will be short.
_______________
Match the pea plant with its combination of alleles.
6. purebred dwarf
a. two traits for tall
7. purebred tall
b. one trait for tall and one trait for dwarf
8. hybrid tall
c. two traits for dwarf
9. A recessive trait is represented by a(n) _______________ letter. A dominant trait is represented by
a(n) ___________________ letter.
10. How would you write the 2 traits for a purebred tall plant? ______. A hybrid tall? _______