Genetics: Fundamentals of
Mendelian Genetics
Classical Genetics
Inherited characteristics
are the result of particulate
factors called genes that are
transmitted from generation to
generation. The vehicles of
transmittance are called
chromosomes. These rules were
first described by
Gregor Mendel.
History
• Scientists have looked at animals
and their offspring for years .
• They noticed that offspring
resemble their parents .
– Puppies grow up to look like their
parents, new maple trees grow from
seeds of adult maple trees. Offspring
resemble their parents because of
their HEREDITY .
• For thousands of years, people
thought that the heredity of a living
thing was merely a blend of the
characteristics of its parents.
• Genetics is much more than blending
characteristics!
Gregor Mendel
• In 1866, around the
time of the civil war,
Gregor Mendel was
one of the first to
recognize that cells
passed on traits in
distinct hereditary
units.
• Mendel lived most
of his life in the
Czech Republic.
– After becoming a priest,
he spent several years
studying science and
mathematics at the
University of Vienna.
– After leaving Vienna,
Mendel returned to the
monastery in Czech
Republic and taught at a
local High School. (Yea!)
– Mendel also was a
gardener at the monastery.
Mendel and His Peas
• In 1856, Mendel
performed his first
set of hybridization
experiments with
fantastic garden pea.
Mendel researched his
garden plants until
1868.
• In 1865, Mendel first
reported the results of
some simple genetic
crosses between certain
strains of garden pea.
Mendel’s Work
• Before arriving at the monastery the
previous gardeners had developed
TRUE-BREEDING stocks of pea plants.
• A true-breeding stocks ALWAYS
passes its characteristics to the next
generation.
• An example of a true breeding plant
would be that one would always
produce tall plants with green pods,
while another stocks would always
produce short plants with yellow
pods.
• Mendel took these TRUE-BREEDING and
mated them and observed their offspring.
• Like many plants, pea plants use parts of
their flowers to reproduce.
• One part produces pollen on male parts
called anthers, and another part produces
egg cells on female parts called stigma .
• When pollen fertilizes egg cells on the same
flower the process is called SELFPOLLINATION. In this case, the offspring
only has ONE parent. Plants may also have
two parents.
• Sometimes pollen from the anthers of one
plant fertilizes the stigma of another flower.
This process is called cross-pollination.
• To perform his experiment, Mendel
had to select the pea plants that
mated with each other.
• He needed to prevent the flowers
from self-pollinating and to control
their cross-pollinating.
•He accomplished this task by first
cutting away the male parts of the
flower. (Ow!)
•Then he dusted the flower with pollen
from a second flower.
• To simplify his investigation Mendel chose
to study only seven TRAITS in pea plants.
Seed
Seed
Seed
Pod
Pod
Flower
Plant
Shape
Color
Coat
Shape
Color
Position
Height
Round Yellow Gray
Smooth
Green Axial
Wrinkled Green
Constricted Yellow
Color
White
Terminal
Tall
Short
Mendel’s Success
• One of Mendel’s
successes was that he
studied just a small
number of traits.
• Also, the traits Mendel
studied had contrasting
forms only. For example,
seed shape is either round
or wrinkled and pod color
is either green or yellow.
Using true-breeding stocks for each of the
seven traits, Mendel crossed pea plants that
showed one form of a trait with pea plants
that showed the other form.
The offspring of these crosses are called
HYBRIDS. These are the offspring of parents
with different characteristics .
Mendel called the hybrids the F1 cross.
The letter F stands for FILIUS, which is
Latin for“SON” . Mendel called the truebreeding plants the P generation. This
stands for the Latin word parentis, meaning“
of the parent”.
What Mendel Expected:
• Mendel expected the traits of the
parents to BLEND in the offspring.
• Instead the traits of just one parent
appeared in the offspring. The traits
of the other parent seemed to have
vanished!.
• If Mendel had stopped with the F1.
cross, he might not be remembered
today.
• But he was curious about what had
happened to the traits that seemed
to have disappeared in the F1
generation.
• He decided to take the next logical
step. He crossed the plants of the F1
among themselves.
Mendel’s Second Cross
• The second cross he called F2
generation.
– In each of the seven traits, the form that
had vanished in the F1 reappeared in the F2
generation.
• These not only reappeared, but
reappeared in noticeable ratios.
– The traits reappeared in approximately
one fourth of the plants in the F2
generation.
– Because the traits did not blend,
Mendel reasoned that some unit of
inheritance must determine each of the
traits he investigated .
Mendel called this unit a MERKMAL ,
the German word for “Character”.
Today the unit that determines traits is
called a GENE.
Units of Inheritance:
• How many copies of each gene does a pea
plant contain?
– Mendel concluded that for each of the seven
traits he investigated, a pea plant must contain
at least at least TWO genes, one from each
parent.
• Today different forms of a gene are
called ALLELES ( uh-LEELZ).
• An example of this is the trait for
height in pea plants. One allele
produces a tall plant while another
allele produces a short plant.
• Some genes may have more than two
alleles, many genes have three , four
, or even dozens of alleles.
Hey, kids! Genetics is fun!
MENDEL’S CONCLUSIONS
MENDEL’S CONCLUSIONS
• After he crossed two plants, one pure
for yellow pod and one pure for green
pod, he discovered the F1 generation
that demonstrated only the green pod
characteristic.
• Not until he self-pollinated the F1
generation did he observe the yellow
pod characteristic in the population.
• He noted a three-to-one ratio in the F2
generation.
• After studying thousands of crosses
Mendel concluded that there are three
principles that govern
inheritance…
Principle One: Dominance &
Recessiveness
• One factor in a pair may mask the other
factor, preventing it from having an
effect. Mendel called the one factor
DOMINANT since it masked or
dominated the other trait. He called
the other factor RECESSIVE.
Principle Two: Segregation
• To explain why traits disappeared in the F1
generation and reappeared in the F2
generation, Mendel reasoned that if each
parent had two factors, each offspring
must have two factors . Each parent must
not be passing two factors because the
offspring would have four.
The two factors must separate during the
formation of egg or sperm.
Principle Three:
Independent Assortment
Mendel also noted that traits for
dominate factors did not appear
together. Green seed pod (a
dominate trait) may appear with a
recessive trait of short height.
Mendel concluded that factors
for different characteristics
were not connected. Factors for
different characteristics are
distributed to reproductive cells
independently.
• Today biologists represent a
dominant allele with a capital letter
and a recessive allele with a
lowercase letter. Thus for pea
plants, a capital letter T represents
the allele for tallness and a lowercase
letter t represents the allele for
shortness.
The End!