Gregor Mendel conducted experiments with pea plants in the mid-19th century to study inheritance of traits. He found that traits are passed from parents to offspring through discrete units called genes, and that these genes segregate and assort independently during reproduction. Mendel identified laws of inheritance, including that each organism inherits two alleles for each gene, and that alleles segregate so each gamete receives one allele. His work formed the basis of classical genetics.

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Basic principles of Inheritance (Mendelism)
Genetics
It is a branch of biology concerned with the study of genes, genetic variation,
and heredity in organisms.
Genomics
It is a branch of biology concerned with the study of whole set of chromosomes in
an organism/species.
Heredity had been observed by Gregor Mendel, a scientist working in the 19th
century, was the first to study genetics scientifically. Mendel studied "trait
inheritance", patterns in the way traits are handed down from parents to offspring.
He observed that organisms (pea plants) inherit traits by way of discrete "units of
inheritance". This term, still used today, is a somewhat ambiguous definition of
what is referred to as a gene.
Mendelian and classical genetics
The observation that living things inherit traits from their parents has been used
since prehistoric times to improve crop plants and animals through selective
breeding. The modern science of genetics, seeking to understand this process,
began with the work of the Gregor Mendel in the mid-19th century.
Prior to Mendel, Imre Festetics, a Hungarian noble, who lived in Kőszeg before
Mendel, was the first who used the word "genetics." He described several rules of
genetic inheritance in his work “The genetic law of the Nature”. His second law is
the same as what Mendel published. In his third law, he developed the basic
principles of mutation.

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Other theories of inheritance preceded Mendel's work. A popular theory during the
19th century, and implied by Charles Darwin's 1859 On the Origin of Species,
was blending inheritance: the idea that individuals inherit a smooth blend of traits
from their parents.
Blending inheritance leads to the averaging out of every characteristic, which as
the engineer Fleeming Jenkin pointed out, blending inheritance makes evolution by
natural selection impossible.
Mendel's work provided examples where traits were definitely not blended after
hybridization, showing that traits are produced by combinations of distinct genes
rather than a continuous blend. Blending of traits in the progeny is now explained
by the action of multiple genes with quantitative effects. Another theory that had
some support at that time was the inheritance of acquired characteristics: the belief
that individuals inherit traits strengthened by their parents. The inheritance of
such a characteristic means its reappearance in one or more individuals in the next
or in succeeding generations. This theory (commonly associated with Jean-Baptiste
Lamarck) is now known to be wrong—the experiences of individuals do not affect
the genes they pass to their children, although evidence in the field
of epigenetics has revived some aspects of Lamarck's theory. Other theories
included the pangenesis of Charles Darwin (which had both acquired and inherited
aspects) and Francis Galton's reformulation of pangenesis as both particulate and

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inherited. It describes that each part of the body continually emitted its own type of
small organic particles called gemmules that aggregated in the gonads, contributing
heritable information to the gametes
Morgan's observation of sex-linked inheritance of a mutation causing white eyes
in Drosophila led him to the hypothesis that genes are located on chromosomes.
Mendel Studied seven traits of pea plant
Our understanding of how inherited traits are passed between generations comes
from principles first proposed by Gregor John 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 color, hair color and even tongue-rolling ability etc.
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)

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 Pod shape (inflated or constricted)
 Pod colour (yellow or green).
Why Mendel preferred Pea plant?
1. It is easy to cultivate
2. Its flowers are hermaphrodite
3. It is normally self-fertilizing but can be cross fertilized
4. Time gap between generations is very short
5. Pea has many distinct traits
Gene: Basic unit of biological information
Locus: Position of a Gene on chromosome
Alleles: Partners of the gene pair on homologous chromosomes
Homologous Chromosomes: A pair of chromosomes having alleles of a gene

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Phenotype: Form of appearance of a trait.
Genotype: Genetic compliment of an individual
Gene Pool: All the genes / alleles found in a breeding population at a given time
are collectively called gene pool
Dominant Allele: Allele of a gene that shows effect of expression on its partner
allele on homologous chromosome
Recessive Allele: Allele of a gene whose expression is affected by its partner allele
on homologous chromosome
Mendel’s Law of Segregation
“Two coexisting alleles for each trait in an individual segregate from each other
during meiosis so that each gamete receives only one of two alleles”
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

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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).
Mendel’s Law of Independent Assortment
“When two contrasting pairs of traits followed in the same cross, their alleles
assort independently into the two gametes”
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).