Genes, Heredity, Law of dominance, Law of segregation, Genotype, Phenotype and variation/mutation

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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