Mendelian principles on inheritance

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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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