Mendel’s Work
Gregor Mendel
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Gregor Mendel was a young priest
from a monastery in Central
Europe where he cared for the
garden and began experimenting
with pea plants.
He noticed that the pea plants had
different physical characteristics,
or traits. Tall plants and short
plants. Green seeds and yellow
seeds.
Gregor Mendel
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He also noticed that the characteristics of the
pea plants resembled those of the parent
plants. This passing of traits from parents to
offspring is called heredity.
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Mendel worked with thousands of pea plants
to understand the process of heredity.
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Mendel’s work formed the foundation of
genetics, the scientific study of heredity.
Why Pea Plants?
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Mendel studied pea plants
because they were selfpollinating.
This means that the pollen from
one flower lands on the pistil of
the same flower.
He developed a method to crosspollinate by removing the pollen
from one plant and brushing onto
a second plant.
Mendel’s Experiments
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Mendel started his experiment
with purebred plants.
A purebred plant is one that
always produces offspring with
the same form of a trait as the
parent.
A purebred short plants always
produce short offspring.
Mendel’s First Experiment
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Mendel first crossed a
purebred short plant with a
purebred tall plant. He called
these two plants the parental
(P) generation.
He called the offspring the first
filial generation or the (F1)
generation.
To Mendel’s surprise, all the
offspring were tall.
Mendel’s Second Experiment
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Mendel then took the
offspring in the F1 generation
and crossed them.
These results also surprised
Mendel.
The offspring in the second
filial generation or (F2) were
a mix of tall and short plants.
He found that 75% were tall
and 25% were short.
Genes and Alleles
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Mendel also tested many other traits in pea plants.
The factors that control traits are called genes.
Different forms of a gene are called alleles
For example the gene for stem height can be
determined by a short allele or tall allele.
Dominant and
Recessive Alleles
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Individual alleles control the inheritance of traits.
Some alleles are dominant and others are
recessive.
The dominant allele is one whose trait always
shows up in the organism when the allele is
present.
A recessive allele is one that will be masked
whenever the dominant allele is present and will
only show up if there are two recessive alleles.
Using Symbols for
Dominant and Recessive Alleles
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In order to represent the two alleles in a
genetic cross, scientists use a shorthand
method for dominant and recessive.
Instead of using words like “tall stems”
they use letters.
Dominant alleles are represented with a
capital letter. Tall stems = (T)
Recessive alleles are represented with
the same letter, only its lowercase. Short
stems = (t)
Understanding Mendel’s Crosses
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Mendel’s P generation plants were
purebred, meaning they had two of the
same alleles.
(TT) = “purebred tall” (tt)= “purebred short”
The F1 offspring received one allele from
each parent making the allele combination:
(Tt)
A hybrid is an organism that has both the
dominant and the recessive allele for a
trait.
Mendel’s Contribution
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Many scientists thought that
Mendel’s explanation of
heredity was oversimplified.
Because of this, his work
was forgotten for 34 years.
However years later, three
other scientists confirmed
his findings and he became
known as “The Father of
Genetics”