• Johann Gregor Mendel was a Moravian
• Born in 1822 in Hyncice, Czechoslovakia on July 22nd.
• His father was a peasant and his grandfather was a gardener.
Mendel was initially taught by a local priest but later on he
was admitted in an Institute of Philosophy in Olmutz.
• But he was not financially well to do therefore in 1843, he
terminated his studies and went back to the monastery in
• Mendel thought that monastery was the best place for him to
study without worrying about how he’d finance his studies. He
was made in charge of the garden at the monastery and
named himself Gregor. He became a priest in 1847. After four
years he went to University of Vienna where he studied
physics, mathematics, chemistry, and botany.
• When he returned to the monastery after completing his
studies, he took a position as a teacher of natural sciences at
the Technical School at Brno.
The experiments carried out
by Gregor Mendel
• He formulated the principles of Inheritance by carrying out
experiments with garden peas.
• Mendel chose garden peas because
they were easy to grow,
produced new generations quickly
peas had easily distinguishable characteristics
was also able to strictly control the breeding patterns of his peas.
Mendel examined the following seven characteristics
found in peas:
• Flower colour - purple or white
• Flower position - axial or terminal
• Seed colour - yellow or green
• Seed shape - round or wrinkled
• Pod shape - inflated or constricted
• Pod colour - green or yellow
• Stem height - tall or short.
• Mendel needed to control the types of fertilisation.
• Experiments were done on both self-pollinated and
cross-pollinated pea plants.
• Self-fertilization was ensured by placing a bag over
the flowers to make sure pollen from the stamens
lands on the carpel of the same flower.
• Cross-fertilization was ensured by cutting off
stamens from a flower before pollen was produced,
then dusting the carpel of the flower with pollen
from another plant.
• To ensure reliability, Mendel used thousands of
plants in each experiment.
• Mendel worked with true-breeding plants:
self-fertilised plants which produced all
offspring identical to the parents.
• Mendel first cross-fertilized two truebreeding plants for one characteristic,
• for example tall plants were crossed with
• The offspring produced are called F1 (1st filial)
• The F1 generation were then self-fertilised or
cross-fertilised to produce a second
Each of the seven traits that Mendel studied had a dominant and a recessive
factor. When two true-breeding plants were crossed, only the dominant factor
appeared in the first generation. The recessive factor appeared in the second
generation in a 3:1 (dominant : recessive) relationship.
This 3:1 ratio occurs in later generations as well. Mendel realized that this
underlying regularity was the key to understanding the basic mechanisms of
• He came to three important conclusions from these
• the inheritance of each trait is determined by "units" or "factors"
that are passed on to descendants unchanged. These units are
now called genes.
• an individual inherits one such unit from each parent for each
• a trait may not show up in an individual but can still be passed on
to the next generation.
• Mendel formulated 3 principles of genetics with
his results of his experiments.
• They are:
• The Law of Dominance
• The Law of Segregation
• The Law of Independent Assortment
Law of Dominance
• In a cross of parents that are pure for contrasting
traits, only one form of the trait will appear in the
next generation. Offspring that are hybrid for a
trait will have only the dominant trait in the
• While Mendel was crossing his pea plants, he noticed
something interesting. When he crossed pure tall plants
with pure short plants, all the new pea plants were
tall. Similarly, crossing pure yellow seeded pea plants
and pure green seeded pea plants produced an F1
generation of all yellow seeded pea plants. The same
was true for other pea traits.
• The results genotype were more like – Tt, Tt, Tt, Tt
• So what was observed is that when there is a dominant
trait present in a genotype, the phenotype is affected
by the dominant allele and not the recessive one.
Law of Segregation
• During the formation of gametes (eggs or
sperm), the two alleles responsible for a
trait separate from each other. Alleles for
a trait are then "recombined" at
fertilization, producing the genotype for
the traits of the offspring.
• Now, when completing a Punnet Square, we model this "Law of
Segregation" every time.
• When you "split" the genotype letters & put one above each
column & one in front of each row, you have SEGREGATED the
alleles for a specific trait. In real life this happens during a
process of cell division called "MEIOSIS".
• Meiosis leads to the production of gametes (sex cells), which are
either eggs or sperm.
• You can see from the p-square that any time you cross two
hybrids, 3 of the 4 boxes will produce an organism with the
dominant trait - "TT", "Tt", & "Tt”.
• 1 of the 4 boxes ends up homozygous recessive, producing an
organism with the recessive phenotype - "tt” .
Law of Independent assortment
•Alleles for different traits
are distributed to sex cells
(& offspring) independently
of one another.
• Mendel noticed during all his work that the height of the plant
and the shape of the seeds and the color of the pods had no
impact on one another.
• The different traits seem to be inherited INDEPENDENTLY.
• The genotypes of the parent pea plants will be:
RrGg x RrGg
• "R" = dominant allele for round seeds
• "r" = recessive allele for wrinkled seeds
• "G" = dominant allele for green pods
• "g" = recessive allele for yellow pods
The results from a dihybrid cross are always the same: 9/16
boxes (offspring) show dominant phenotype for both traits
(round & green), 3/16 show dominant phenotype for first
trait & recessive for second (round & yellow), 3/16 show
recessive phenotype for first trait & dominant form for
second (wrinkled & green), & 1/16 show recessive form of
both traits (wrinled & yellow).
• So, as you can see from the results, a green pod
can have round or wrinkled seeds, and the same
is true of a yellow pod.
• The different traits do not influence the
inheritance of each other. They are inherited
Reasons for Mendel’s success
Mendel was successful because :
• He used peas, which were easily grown
• Peas produced successive generations rapidly
• He selected easily observable characteristics
• strictly controlled the fertilization process
• He used mathematics rigorously to analyze his results
• used large numbers of plants.
• He studied traits that had two easily identified factors.
• One of the main advantages Mendel had over the other
scientists was that he excelled in mathematics which was
majorly used in his experiments.
• He maintained an accurate record of all the observations.
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