This document discusses Gregor Mendel's experiments with pea plants that established the basic rules of genetic inheritance. Mendel recognized that parental traits were not blended in offspring, as was commonly believed at the time, but were inherited as distinct factors. He conducted controlled crosses using true-breeding pea plants with distinct traits. His experiments showed that traits are inherited based on dominant and recessive alleles that segregate independently, with dominant traits masking recessive ones. This led to predictable phenotypic ratios in offspring of crosses. Mendel's work established the foundations of classical genetics.

1.  INTRODUCTION TO GENETICS
 Mendel’s Laws of Inheritance
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2.  The basic rules of
inheritance were first
demonstrated by Gregor
Johnn Mendel in the
mid-1800s.
 At the time of Mendel’s
work, most thought
that parental traits were
fluids that “blend” in
offspring
 Mendel recognized that
this model did not
explain what he
observed. ANJAN NEPAL

3.  Mendel chose a model
system and carefully
established testing
conditions
 he used pea plants that
he could outcross or
allow to self-fertilize
 he chose traits that had
two clear possible
outcomes (yellow or
green seeds, etc.)
 he established true-
breeding or “pure” lines
to use for genetic crosses
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4.  P generation (or P1) = parental generation
 F1 generation = first generation offspring (from filial)
 F2 generation = second generation offspring
 phenotype – appearance or characteristic of an
organism
 genotype – genetic makeup of an organism,
determines phenotype
 gene – unit of heredity; controls a trait that
determines a phenotype
 locus – the location of a particular gene on a
chromosome
 alleles – alternative versions of a gene
 dominant – allele that dominates over others in
determining phenotype
 recessive – allele whose phenotypic expression is
“hidden” when a dominant allele is present
 hybrid – offspring from a cross between two “pure”
lines of different, competing phenotypes
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6.  The characters which are expressed by hybrids are
called Dominant character.
 The suppressed character is called recessive character.
 For example, if cross is made between Tall pea plant
(TT) and dwarf pea plant (tt), all the hybrids in F1
generation were tall.
 Here, the expressed tallness is dominant character and
the masked or suppressed dwarfishness is recessive
character.
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7. Contains large number of contrasting character
Very short life cycle
Suited for controlling breeding i.e. both self and cross pollinatedANJAN NEPAL

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9.  Each characteristics of an
organism is determined by
a pair of factors of which
only one can be present in
each gamete.
 When a pair of contrasting
characters are brought
together in a hybrid (F1),
the member of the pair stay
together without mixing
and separate when the
hybrid form gametes.
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10.  when F1 plants were
crossed or selfed, the F2
plants had both P1
phenotypes in a ratio of
roughly 3:1.
 3 purple flower: white
flower
 So, recessive traits are
not lost in a mixing of
parental phenotypes –
they are merely hidden
in some “carrier”
individuals. ANJAN NEPAL

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12.  Phenotype is the actual appearance or
characteristic, and is determined by genotype.
 knowing the phenotype will not always directly
reveal the genotype (recessive traits can be
masked)
 Genotype is the genetic composition of an
organism or actual alleles of a character.
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13. Genotypes are either homozygous or heterozygous
 Homozygous – The homologous chromosomes that
have the same allele at the locus. The trait from that
allele will be expressed phenotypically.
 Heterozygous – The homologous chromosomes that
have different alleles at the locus. If there is a
dominant allele, the trait of the dominant allele will
be expressed phenotypically.
 The same letter is used to indicate all alleles of
same trait
 DOMINANT ALLELES ARE CAPITALIZED; recessive alleles are
lowercase
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14. Monohybrid cross
 Cross between two
parents with one pair of
contrasting characters
 Ex: Cross between Tall
pea plant (TT) and dwarf
pea plant (tt)
 Punnett square
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15. Monohybrid cross
Phenotypic ratio
= 3:1
Genotypic ratio
=1:2:1
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16.  Dihybrid cross – Cross between individuals that are both
heterozygous for two different genes that you are
following
 When Mendel performed Dihybrid crosses he found
phenotype ratios of 9:3:3:1
 note that this follows the product rule:
3:1 x 3:1 = 9:3:3:1
 Segregation of any one pair of
alleles is independent of the
segregation of other pairs of alleles
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21. Mutation
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22. Down’s
syndrome
Sickle Cell
anemia
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