Gregor Mendel conducted experiments with pea plants in the 1860s to develop an understanding of heredity. He studied seven traits in pea plants and found that traits are passed to offspring through discrete factors, now called genes. His experiments led him to formulate the laws of inheritance, including the law of dominance, the law of segregation, and the law of independent assortment. Mendel's principles explained inheritance in a way that was ahead of scientific understanding at the time and formed the foundation of modern genetics.

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Amity Institute of Biotechnology
ADVANCED CELL BIOLOGY AND GENETICS
MSB: 104
Credit Units: 03
Dr. Pallavi Singh Chauhan
Assistant Professor

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Mendelian principles on inheritance
• Our understanding of how inherited traits are passed between
generations comes from principles first proposed by Gregor
Mendel in 1866.
• Mendel worked on pea plants, but his principles apply to traits
in plants and animals – they can explain how we inherit our
eye colour, hair colour and even tongue-rolling ability.
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Gregor Mendel
Gregor Mendel (1822–1884) is known as the father of genetics. He proposed
the key laws of genetics from this work on inheritance of traits in peas in 1866.
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Inheritance in pea plants
• Mendel followed the inheritance of 7 traits in pea plants
(Pisum sativum). He chose traits that had 2 forms:
• Pea shape (round or wrinkled)
• Pea colour (yellow or green)
• Flower colour (purple or white)
• Flower position (terminal or axial)
• Plant height (tall or short)
• Pod shape (inflated or constricted)
• Pod colour (yellow or green).
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• Mendel began with pure-breeding pea plants because they
always produced progeny with the same characteristics as the
parent plant.
• Mendel cross-bred these pea plants and recorded the traits of
their progeny over several generations.
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• Mendel’s principles of inheritance
Key principles of genetics were developed from
Mendel’s studies on peas.
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Why was Pea Plant Selected for Mendel’s Experiments?
• He selected a pea plant for his experiments:
• The pea plant can be easily grown and maintained.
• They are naturally self-pollinating but can also be cross-
pollinated.
• It is an annual plant, therefore, many generations can be
studied within a short period of time.
• It has several contrasting characters.
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• Mendel conducted 2 main experiments to determine the laws
of inheritance. These experiments were:
• Monohybrid Cross Experiment
• Dihybrid Cross Experiment
• While experimenting, Mendel found that certain factors were
always being transferred down to the offspring in a stable way.
Those factors are now called genes i.e. genes can be called as
the units of inheritance.
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• Mendel experimented on a pea plant and considered 7 main
contrasting traits in the plants. Then, he conducted both the
experiments to determine the a forementioned inheritance
laws.
• A brief explanation of the two experiments is given below:
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Monohybrid Cross
• In this experiment, Mendel took two pea plants of opposite
traits (one short and one tall) and crossed them.
• He found the first generation offsprings were tall and called it
F1 progeny.
• Then he crossed F1 progeny and obtained both tall and short
plants in the ratio 3:1.
• Mendel even conducted this experiment with other contrasting
traits like green peas vs yellow peas, round vs wrinkled, etc. In
all the cases, he found that results were similar.
• From this, he formulated the laws of Segregation And
Dominance.
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Dihybrid Cross
• In a dihybrid cross experiment, Mendel considered two traits,
each having two alleles.
• He crossed wrinkled-green seed and round-yellow seeds and
observed that all the first generation progeny (F1 progeny)
were round-yellow.
• This meant that dominant traits were the round shape and
yellow colour.
• He then self-pollinated the F1 progeny and obtained 4 different
traits wrinkled-yellow, round-yellow, wrinkled-green seeds
and round-green in the ratio 9:3:3:1.
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Conclusions from Mendel’s Experiments
• The genetic makeup of the plant is known as the genotype. On
the contrary, the physical appearance of the plant is known as
phenotype
• The genes are transferred from parents to the offsprings in
pairs known as allele.
• During gametogenesis when the chromosomes are halved,
there is a 50% chance of one of the two alleles to fuse with the
other parent.
• When the alleles are same they are known as homozygous
alleles when the alleles are different they are known as
heterozygous alleles.
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Mendel’s laws
• The two experiments lead to the formulation of Mendel’s laws
known as laws of inheritance which are:
• Law of Dominance
• Law of Segregation
• Law of Independent Assortment
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Law of Dominance
• Mendel found that paired pea traits were either dominant or
recessive.
• When pure-bred parent plants were cross-bred, dominant traits
were always seen in the progeny, whereas recessive traits were
hidden until the first-generation (F1) hybrid plants were left to
self-pollinate.
• Mendel counted the number of second-generation (F2)
progeny with dominant or recessive traits and found a 3:1 ratio
of dominant to recessive traits.
• He concluded that traits were not blended but remained
distinct in subsequent generations, which was contrary to
scientific opinion at the time.
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Law of Segregation
• Mendel proposed that, during reproduction, the inherited
factors must separate into reproductive cells.
• He had observed that allowing hybrid pea plants to self-
pollinate resulted in progeny that looked different from their
parents.
• Separation occurs during meiosis when the alleles of each gene
segregate into individual reproductive cells (eggs and sperm in
animals, or pollen and ova in plants).
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Principle of independent assortment
• Mendel observed that, when peas with more than one trait
were crossed, the progeny did not always match the parents.
This is because different traits are inherited independently –
this is the principle of independent assortment.
• For example, he cross-bred pea plants with round, yellow
seeds and plants with wrinkled, green seeds.
• Only the dominant traits (yellow and round) appeared in the
F1 progeny, but all combinations of trait were seen in the self-
pollinated F2 progeny.
• The traits were present in a 9:3:3:1 ratio (round, yellow: round,
green: wrinkled, yellow: wrinkled, green).
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Exceptions to Mendel’s rules
• There are some exceptions to Mendel’s principles, which have
been discovered as our knowledge of genes and inheritance
has increased.
• The principle of independent assortment doesn’t apply if the
genes are close together (or linked) on a chromosome.
• Also, alleles do not always interact in a standard
dominant/recessive way, particularly if they are codominant or
have differences in expressivity or penetrance.
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• Penetrance is used to describe whether or not there is a clinical
expression of the genotype in the individual.
• Expressivity is the term that describes the differences observed
in the clinical phenotype between two individuals with the
same genotype.
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Key Points on Mendel’s Laws
• The law of inheritance was proposed by Gregor Mendel after
conducting experiments on pea plants for seven years.
• The Mendel’s laws of inheritance include law of dominance,
law of segregation and law of independent assortment.
• The law of segregation states that every individual possesses
two alleles and only one allele is passed on to the offspring.
• The law of independent assortment states that the inheritance
of one pair of genes is independent of inheritance of another
pair.
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