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� � � � � � � � � � � � � � � � � �� � � � �� � � � � � �� � � � � � �� � � � � � � �� � � �
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Principles of Inheritance & Variation - Part I
Sugar-phosphate double helix
Base pair
RNARIBONUCLEIC ACIDDNA
DEOXYRIBONUCLEIC ACID
�Single-stranded genetic material
�Found in some viruses
��� � � � � types depending on function:
mRNA (messenger RNA)tRNA (transfer RNA)rRNA (ribosomal RNA)
Sugar-phosphate helix
NucleobasesNucleobases
GUANINE G
ADENINE A
CYTOSINE CNH2
NH
O
N
THYMINE T
H3CNH
NH
O
O
NH
NH2
NH
O
N
N
H2N
NH
N
N
N
GUANINE G
ADENINE A
CYTOSINE CNH2
NH
O
N
URACIL U
NH
NH
O
O
NH
NH2
NH
O
N
N
H2N
NH
N
N
N
G
G
G
C
C
G
G
T
T
A
A
C
C
G
C
C
G
C
G
C
G
A
U
C
Two Identical ChromatidsOne is exact copy of the otherand each contains one DNA molecule
Short arm structure
Long arm structure
Long string like DNA molecule formed into compact structures by proteins called histones.
Constricted point of the chromosome
p arm
q arm
DNA molecule
Centromere
Gregor Johann Mendel
� �� � � �� � � � �� � � � �
� �� � � �� �� � � � � � � � � � � �� � � � � � � � � �
Proposed
Law
s of
Inhe
rita
nce
� �� � � �� �� � � � � � � � � � �Father of Modern Genetics
� � � � � � � � � �� � � � �� �� � � � � � �� � � � � � � �� � � � � �� � � � � � � �� �� � � � � � � � �
Genetic Material
Genetic Inheritance
� � �� � � � � �� �� � � � � �� � � � � � � � � � �
Gregor Johann Mendel
� � � � � � � � � � � �� ���� � � �� � � � � � � � �
�Most common genetic material, except
in some viruses
�Double stranded genetic material
�Nucleotides create chain via
phosphodiester bonds & nitrogen bases form
hydrogen bonds
�DNA replicates by forming RNA intermediates
F1 generation F2 generation
� � � �� � � � �� � � � � � � � �� �� � � � � � � � � � � � � � �� �� � � � � � � � � � � �� � � � � � � � � �
In heterozygous condition, thedominant allele gets expressed. F1 generation express dominant alleles.
Two alleles do not mix when they come together in hybrid pair & are independent of each other.
During gamete formation, two genes segregate independently of each other as well as of the other trait
�A monk who went to University of
Vienna to study science & Mathematics
�1856 – 1863: Conducted experiments
on pea plant (Pisum sativum)
�Studied the pattern of inheritance
�Proposed laws of inheritance
�Plants are easy to grow & maintain
�Several visibly distinct & contrasting
characters
�Annual plant
�Naturally self-pollinating but can be
cross-pollinated
�Genes exist as alleles that pass from parents to o�spring
�Homozygous: 2 same alleles
�Heterozygous: Di�erent alleles
�Dominant gene: TT (capital alphabet); Recessive gene: tt (small alphabet)
�Phenotype: Physical appearance
�Genotype: Genetic makeup
�50% chance of either allele to fuse with other parent to form zygote
�Cross-pollinated 2 pure lines for contrasting characteristics to obtain F1 generation �Self-pollinated F1 generation that gave rise to the F2 generation
Gregor Johann Mendel (1822 - 1884)
A monk who went to University of Plants are easy to grow & maintain
� � � � � � �� �� � � � � � �� � � � � � � � � � � � � � � �� � � �� � � � � � � � � �� � � �
Seven Contrasting Characters of Pea Plant Studied by Mendel
Results Conclusion
Laws of Inheritance
Mendel’s Experiment
FlowerColour
SeedColour
PodColour
FlowerPosition
SeedShape
PodShape
PlantHeight
DominantTraits
RecessiveTraits
Purple
Green TerminalWrinkledShortWhite
Pea plant(Pisum sativum)
All tall plantsTall Short Short
1 3
Yellow
Yellow Constricted(�at)
Round Green AxialIn�ated(full)
Tall
TT
Tt
Xtt
Parents
F1 generation
TT
Tt
Xtt
TT Tt Tt tt
Parents
F1 generation
Parental (P)generation
F1 generation
Gametes
RrYy
x
rryyRRYY
ryrYRyRY
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Principles of Inheritance & Variation - Part II� � � � � � � � � � � � �� �� � � � � �
Gregor Johann Mendel
Morgan’s Experiment
� � � � � � �
Crossing over
Principles of Inheritance & Variation - Part III
Linkage & recombination are the phenomena which describe the inheritance of genes
�Two or more linked genes are always inherited together in the same combination for over two generations
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Homologous chromosomes
aligned
ABC
ABC
ABC
ABC
ABC
abc
abc
abc
abc
abc
Chromosomescrossover
Recombinantchromosomes
Non-recombinantchromosomes
Parental phenotypes
Total ParentalPhenotypes: 1,541
Total recombinantPhenotypes: 900
Total progeny: 1,541 + 900 = 2,441Percent recombinants: 900/ 2,441X 100 = 36.9
white eyesminature wings
white eyeswild type wings
wild type eyesminature wings
white eyesminature wings, o+
F1 phenotypes
o+ wild type
wild type
F2 phenotypes
�
wild type �
white eyesminature wings, �
Total: 750
o+ 359 391�
Total: 791
439 352o+ �
Total: 455
218 237o+ �
Total: 455
235 210o+ �
w mw m( ) w+ m+
( )
whitewhite
Sex Determination: System that decides the sexual characteristics of an organism
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Male: XXFemale: XY
Female: ZW Male: ZZ
Mutation & Chromosomal Disorders: Major drawbacks of genetic inheritance
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