This document discusses principles of inheritance and variation. It begins by defining key terms like genetics, heredity, and variation. It then discusses Gregor Mendel's experiments with pea plants in the 1800s, which formed the basis of modern genetics. Mendel discovered the laws of inheritance, including dominance, segregation, and independent assortment. The document also covers variations, including continuous vs discontinuous and somatic vs germinal variations. It discusses inheritance of multiple genes, including incomplete dominance, codominance, and polygenic inheritance. Overall, the document provides a comprehensive overview of classical genetics concepts and terminology.

1. PRINCIPLES OF
INHERITANCE
AND VARIATION
BY: DR. PRITIMA GUPTA

2. INTRODUCTION
• Branch of biology that deals with the study of genes, genetic
variation and hereditary is known as GENETICS.
• INHERITANCE- the process by which characters are passed
on from parent to progeny.
• VARIATION- It is the degree by which progeny differ from
their parents.

3. TERMINOLOGIES
• GENETICS – It is the branch of life science that deals with the study of
heredity and variation.
• HEREDITY – It is the transmission of characters from parents to their
offsprings.
• VARIATION – It is the difference among the offsprings and with their
parents.
• HEREDITARY VARIATIONS – It these are genetical and inheritable.
• ENVIRONMENTAL VARIATION – It these are acquired and non
inheritable.

4. GREGOR JOHANN MENDEL
• HE IS THE FATHER OF GENETICS.
• FATHER OF MODERN GENETICS.
• Gregor Mendel developed the principles
of heredity while studying seven pairs of
inherited characteristics in pea plants.
• He conducted experiments on for 7
years from 1856-1863 & gave laws of
inheritance.
• HIS APPROACH – Used pea plant for
his experiments due to -
1. Pure variety are available.
2. Pea plants are easy to cultivate.
3. Life cycle of plants are only few
months. So that result can be got early.
4. Contrasting trait are observed.
5. Flowers are bisexual and normally self
pollinated.
6. Flowers can be cross pollinated only
manually.
7. Hybrids are fertile.
8. Pure line breeding was possible.
MENDEL’S EXPERIMENT

5. 1. Applied statistical analysis
& mathematical logic for
biology problems.
2. Large sampling size.
3. Confirmation of inference
from experiments on
successive generations of
test plants, proved
general rules of
inheritance.
TRAITS OF PEA
PLANT
CHARACTERISTIC DOMINANT RECESSIVE
HEIGHT Tall Dwarf
SEED SHAPE Round Wrinkled
SEED COLOUR Yellow Green
SEED COAT
COLOR
Green Yellow
POD SHAPE Inflated (full) Constricted (dull)
POD COLOUR Green Yellow
FLOWER
POSITION
Axial Terminal
DRAWBACK OF
EXPERIMENT

6. INHERITANCE OF ONE GENE/
MONOHYBRID CROSS
MONOHYBRID CROSS –Different
genes are controlling the same
characters.
 F1 – Filial 1 generation
 F2 – Filial 2 generation
INFERENCE –
1. F1 & F2 neither showed blending.
2. Only 1 parental trait expresses itself
in F1 generation.
3. Both the traits got expressed in F2
generation in the ratio 3:1.
4. No blending was seen in either
generation.

7. 5. F2 generation- 1/4th were dwarf & 3/4th tall- identical to parents.
 Mendel proposed- Something is stably being passed to the next generation
through gametes ‘FACTORS’ – GENES.
 GENES/FACTORS – Unit of inheritance, contain the information required to
express particular trait.
 Genes which code for pair of contrasting trait- ALLELES.
 Alphabetical symbols were used; T-Tall, t- dwarf.
 Mendel proposed- true breeding tall or dwarf plant- IDENTICAL OR
HOMOZYGOUS ALLELE pair of TT or tt (genotype).
 Mendel found phenotype of HETEROZYGOTE Tt of F1 was same as parent with
TT & proposed, in a pair of dissimilar factors.
 One dominates the other & hence called dominant (T) & recessive (t)

8. PUNNETT SQUARE
• It is a graphical representation to
calculate the probability of al
possible genotypes of an organism
in a genetic cross.
• It was given by REGINALD C.
PUNNETT.
• Self- pollination- 50%.
• F2- 3/4th tall & 1/4th Dwarf
phenotypically.
• ¼ : ½ : ¼ ratio of TT: Tt: tt
genotype.
• Binomial equation – (ax + by)2
where ax = ½ T & by = ½ t.
• On putting the values & solving
this equation, we get – ¼ TT + ½
Tt + ¼ tt.
• Which implies that among that
cross 25% is TT = tall plants
50% is Tt = tall plants
25% is tt – dwarf plants.
• So thereby we get the genotype
ratio as 1:2:1 i.e. TT: Tt: tt.

9. TEST CROSS
• The cross
between
hybrid and its
homozygous
recessive
parent is
called test
cross. It is
used to
identify the
genotype of
the hybrid.

10. LAWS OF INHERITANCE
• LAW OF DOMINANCE – First law of Inheritance.
• LAW OF SEGREGATION – Second law
LAW OF DOMINANCE –
1. Characters are controlled by factors.
2. Factors occurs in pairs.
3. In a dissimilar pair of factors one member of the pair dominates (dominant) the
other (recessive).
• Used to explain the expression of only one of the parental characters in
monohybrid cross (F1) & expression of both in F2.
• Also explains proportion 3:1 in F2

11. LAW OF SEGREGATION –
1. It states that, ‘when a pair of factors for a character brought together in a
hybrid, they segregate (separate) during the formation of gametes.
2. Alleles do not blend & both characters recovered in F2 & one in F1.
3. Factors which is present in parent segregate & gametes receives only one of
two factors.
4. Homozygous parent- one kind gamete.
5. Heterozygous parent- two kind gamete each one have one allele with equal
proportion.

12. INCOMLETE DOMINANCE
• The inheritance in which allele for a
specific character is not completely
dominant over other allele is called
Incomplete dominance.
• It is also called PARTIAL DOMINANCE
OR SEMI DOMINANCE.
• Ex: Snapdragon or Antirrhinum sp.
• Cross between true breed red flower
(RR) & white flower (rr), F1 generation-
Pink (Rr) & after self pollination in F2
generation- 1 (RR) Red: 2 (Rr) Pink: 1
(rr) white.
• Genotype – 1: 2: 1 i.e. RR: Rr: rr
• Phenotype – 1: 2: 1 i.e. Red: Pink:
White.

13. PARENT RR Rr
GAMETES R r
F1 Rr
ON SELF CROSSING
F1
F2
GAMETES
R r
R
RR
(RED)
Rr
(PINK)
r Rr
(PINK)
rr
(WHITE)
GENOTYPIC RATIO –
1: 2: 1
RR: Rr: rr
PHENOTYPIC RATIO –
1: 2: 1
RED: PINK: WHITE
Ex: Normal allele – Normal Enzyme
Modified allele – i. Normal/ Less
efficient enzyme
ii. Non-functional
enzyme
iii. No enzyme

14. CO - DOMINANCE
• Both the alleles for a character
are dominant and express its full
character is called co-dominance.
• Ex: AB blood group of human
being.
• Blood group in humans are
controlled by 3 alleles of a gene I.
They are IA IB and i.
• IA is responsible for production of
antigen –A.
• IB is responsible for production of
antigen –B.
• i does not produces any antigen.

15. MULTIPLE ALLELISM
• When two or more alleles governs the same character/ expresses the same
character is c/as MULTIPLE ALLELISM.
• Ex: ABO blood grouping. Three alleles govern same character.
• Single gene product may produce more than one effect is k/as PLEIOTROPY.
• Eg- Starch Synthesis in Pea seeds- controlled by a gene having two allele B & b
• BB = Starch synthesis effective, grain size – large, seed shape – round.
• bb = Lesser efficiency in starch synthesis, grain size – small, seeds – wrinkled.
• Bb = Intermediate.

16. INHERITANCE OF TWO GENES
• LAW OF INDEPENDENT
ASSORTMENT – When two
pairs of traits are combined in
a hybrid, segregation of one
pair of characters is
independent of the other pair
of characters.
OR
• It states that, ‘factors for
different pairs of contrasting
characters in a hybrid assorted
(distributed) independently
during gamete formation.
PHENOTYPIC RATIO – 9: 3: 3: 1
Round Yellow: Round Green: Wrinkled Yellow: Wrinkled Green
GENOTYPIC RATIO – 1: 2: 2: 4: 1: 2: 1: 2: 1
RRYY: RRYy: RrYY: RrYy: Rryy: Rryy: rrYY: rrYY: rryy

17.  Mendel work published
in 1865 but remain
unrecognized till
1900.
 Reasons for that:
1. Lack of
communication.
2. Concept of genes /
factors- clear.
3. Mathematical
approach for biology
was not accepted.
4. No proof for
existence of factors
MENDEL’S WORK
RECOGNITION

18. VARIATIONS
VARIATION
As per nature of
cells it affects
SOMATIC
GERMINAL OR
BLASTOGENIC
As per degree of
differences
produced
CONTINUOUS
DISCONTINUOUS
• The differences shown by the individual of a species and also by the offspring of
the same parents are referred to as variations.

19. SOMATIC VARIATIONS
SOMATIC
VARIATIONS
CAUSES
ENVIRONMENTAL
USE & DISUSE OF
ORGANS
CONSCIOUS
EFFORTS
• Somatic variations are those which are not inherited
from parents & affects only the somatic cells. Also
k/as ACQUIRED VARIATIONS.
1. Nutrition
2. Habitat
3. Better conditions
4. Water
1. Weakening of teeth
3. Muscular body
2. Left handedness of
a person
1. Castration
2. Mutilation
3. Small feet etc.

20. GERMINAL VARIATIONS
• Variations are caused due
to germ cells and thus
inheritable.
• Eg: haemophilia, blood
groups, colour blindness,
baldness, eye colour etc.
• Crossing over
• By radiations
• By polyploidy
• Change in chemical nature
of genes
• Modifications in
chromosome structure
GERMINAL
VARIATIONS

21. CONTINUOUS VARIATIONS/
FLUCTUATING VARIATIONS
• Such variations are
small and indistinct
when compared to
normal.
• Unstable and non-
inheritable.
• Produces a bell
shaped graph.
CONTINUOUS
VARIATIONS
SUBSTANTIVE
Variations which bring
change in size, weight,
colour etc. They affect the
morphology.
MERISTIC
Bring change in certain parts of the
organism e.g. No. of segments get
changed in earthworm, change in no.
of arms in starfish, sepals & petals
no. changed etc.

23. DISCONTINUOUS VARIATIONS
• Such variations are
large and represent
the conspicuous
differences of the
progeny from
parents.
• They are referred as
mutations or sports
or saltations.
• Mutations appear
suddenly and are
inheritable and
stable.
DISCONTINUOUS
VARIATIONS
SUBSTANTIVE
Mutations bring variation
in size, weight, shape,
colour etc. Eg:
brachydactyly, syndactyly
etc.
MERISTIC
Bring change in no. of certain parts
of the organism e.g. polydactyly.

24. SIGNIFICANCE OF VARIATIONS
1. Constitutes the raw material for evolution.
2. Useful variants of animals and plants are produced.
3. Variations form the basis of heredity.
4. Help in adaptations of organisms to changed environment.
5. Make some individuals better suited in the struggle for existence.
6. Variations provide each organism a distinct individuality.

25. POLYGENIC INHERITANCE/
QUANTITATIVE INHERITANCE
• When one character is controlled
by two or more genes.
• Ex: Skin colour in human beings
which is controlled by 3 genes – A,
B, C.
• Quantity of dominant genes
decides the amount of melanin
produced in the body.
• AA BB CC = Dark skin colour
• (max. melanin produced).
• Aa bb cc = Albinos
• (no melanin produced).
• Aa Bb Cc = Wheatish colour
(intermediate melanin).
• Talking about the range –
• 6 Dominant – Black
• 5 Dominant – Very dark
• 4 Dominant – Dark
• 3 Dominant – Intermediate
• 2 Dominant – Fair
• 1 Dominant – Very fair
• 0 Dominant - Albino

26. P AABBCC x aabbcc
ABC abc
F1 AaBbCc
F2 AaBbCc AaBbCc
A
C ABC
B c ABc
b C AbC
c Abc
a
C aBC
B c aBc
b C abC
c abc
ABC ABc AbC Abc aBC aBc abC abc
ABC 6 5 5 4 5 4 4 3
ABc 5 4 4 3 4 3 3 2
AbC 5 4 4 3 4 3 3 2
Abc 4 3 3 2 3 2 2 1
aBC 5 4 4 3 4 3 3 2
aBc 4 3 3 2 3 2 2 1
abC 4 3 3 2 3 2 2 1
abc 3 2 2 1 2 1 1 0
0
5
10
15
20
25
0
5
10
15
20
25
0 1 2 3 4 5 6
BELL GRAPH – POLYGENIC INHERITANCE
Series 1 Column1 Series 3

27. GENE INTERACTION
1. INTER – ALLELIC OR INTRA – GENIC
GENE INTERACTION –
Expression of character is produced
by interaction between alleles of a single
gene. Non allelic gene interaction.
2. NON – ALLELIC OR INTERGENIC
GENE INTERACTION –
Expression of character is produced
by interaction between two or more
genes.
HOMOLOGOUS CHROMOSOME –
Chromosomal pairs which are similar in
length, gene position, centromere location.

28. INTERGENIC EXAMPLES INTRAGENIC EXAMPLES
1. Complementary gene interaction 1. Multiple allelism
2. Supplementary gene interaction 2. Co – dominance
3. Epistasis – Dominant
Recessive
3. Incomplete dominance
4. Duplicate gene interaction
5. Additive gene interaction
6. Inhibitory gene interaction

29. INTERGENIC – 1. COMPLEMENTARY
GENES
• Genes located at d/f loci of same or d/f chromosome but work together to show
some expression.
• Ex: Lathyrus odoratus (Sweet pea)

30. • Two genes +nt on d/f loci but 1 gene expresses itself independently & the other
gene is w/o any expression. But both gene if +nt in dominant form expresses a
third trait/ effect.
• Ex: Coat colour in mice.
• B = black
• A = albino (no colour)
• AB = agouti
• ab = albino
2. SUPPLEMENTARY GENES
RATIO – 9: 4: 3

31. 3. EPISTAXIS/ INHIBITING GENES
• Two genes located at d/f loci & 1 out of 2 genes is dominating/ epistatic & the
other gene get suppressed/ dominated k/as “HYPOSTATIC GENE”.
EPISTASIS
DOMINANT
EPISTASIS
Ex: Cucurbita
pepo
(Summer squash)
12: 3: 1
RECESSIVE
EPISTASIS
Ex: Colour coat in
mice
9: 4: 3

32. 4. DUPLICATE GENE INTERACTION
• When 2 genes located on d/f loci of the same or d/f chromosome but expresses
same phenotype independently (+nt in dominant form).
• Ex: Fruit shape of Capsella bursa (Shepherd’s form).
• Genes +nt on d/f loci of same or d/f chromosomes.
• Both genes shows independency.
• Ex: Comb shape in Fowls.
RATIO – 15: 1
5. COLLABORATOR GENES
RATIO – 9: 3: 3: 1

33. PLEIOTROPIC GENES
• Single gene product may produce more than one effect is k/as PLEIOTROPY.
• Ex: Sickle cell anemia – Autosomal Recessive Disorder.
2 α
2 β
• In one of the β chain there is a mutation
where 6th position amino acid (Glutamic acid)
is replaced by valine.
#CHANGES –
1. Structure hampered of RBC.
2. Decreased O2 carrying capacity.
} Polypeptide
chains
Normal Hb -

34. PARENT HbA x Hbs
F1 HbAHbs
SELFING F1
GENERATION
HbA Hbs
HbA HbAHbA
Normal
HbAHbs
Carrier
Hbs HbAHbs
Carrier
HbsHbs
Diseased
• Individuals with Hb configuration are resistant to malaria because malarial
parasites cannot penetrate & survive in sickle celled RBC’s.
#QUALITATIVE INHERITANCE/ MONOGENIC INHERITANCE –
One dominant allele influences the complete trait.

35. LETHAL GENES
• Lethal means death causing.
• Lethal factors were supported by “CUENOT” – French geneticist – in mice.
PHENOTYPE
AFFECT
VIABILITY GENES
VIABILITY GENES
LETHAL
GENES
DOMINANT
AUTOSOMES
AA/Aa
RECESSIVE
AUTOSOMES
aa

36. CHROMOSOMAL THEORY OF
INHERITANCE
• 1865 – Mendel published his work.
• 1901 – 3 scientists redid his work.
• 1902 – CTI was given by SUTTON &
BOVERI.
• Lets compare the two views –
• 1870 OSCAR HERTWIG –Nucleus
contains theory of inheritance.
• CORRENS – Gave the evidence in
Mirabilis jalapa that cytoplasm also
contains heredity material.
MENDEL SUTTON & BOVERI
1. Factors occurs in pairs. Factors are +nt on chromosomes &
chromosomes also occurs in pairs.
2. 2 forms of a same gene are
allele in pair.
Homologous chromosome.
3. Alleles segregate at the time of
gamete formation.
Chromosomes also separate at the
time of gamete formation.
4. Alleles/ factors assort
independently.
Chromosomes also assort
independently.

37. LINKAGE AND RECOMBINATION
• When genes are closely +nt, they link together in a group and
transmitted as a single unit.
• First reported in Drosophila by T.H MORGAN – 1910.
• If genes are located at extremes – recombination chances are more.
• If genes are located more closely – recombination chances are less.

38. THEORIES OF LINKAGE
1903.
No. of groups
of genes =
no. of
chromosomes
MORGAN
& CASTLE.
Genes are
bound by
chromosomal
material and
are
transmitted as
a whole.
BATESON &
PUNNETT.
1906.
Dominant/recessiv
e alleles remains
together –
COUPLING
PHASE/ CIS.
Dominant/
recessive alleles
remain separated –
REPULSION/
TRANS PHASE.
SUTTON’SHYPOTHESISOFLINKAGE
MORGAN’SHYPOTHESISOFLINKAGE
CHROMOSOMALHYPOTHESISOFLINKAGE
COUPLINGANDREPULSIONHYPOTHESIS
1910.
Genes of
homologous
parents enter in
the same
gamete and
tends to remain
together.
Opposite in
heterozygous
parents.

39. COUPLING &
REPULSION
HYPOTHESIS
Eg: Plants of sweet pea
having blue flowers (BB)
and long pollen (LL) with
red flowers (bb) and round
pollen (ll).
F1 generation – BbLl –
Blue flower & long pollen.

40. TYPES OF
LINKAGE
COMPLETE
LINKAGE
Linked genes do
not get separated.
(Rare)
INCOMPLETE
LINKAGE
Genes will get
separated.
# LINKAGE GROUPS –
 Linkage groups = no. of chromosome in one
set (n).
 No. of linkage group in an organism = no. of
haploid chromosomes +nt in its cells.
 Eg: In humans – n=23 = linkage groups.
# STRENGTH OF LINKAGE  1/ DISTANCE
B/W THE GENES
# FACTORS AFFECTING LINKAGE –
 DISTANCE - ↑ distance ↓ strength.
 AGE - ↑ age ↓ strength.
 TEMPERATURE -↑ temp ↓ strength.
 X-RAYS -↑ X-rays exposure ↓strength.
# STRENGTH OF LINKAGE  1/ CROSSING
OVER.
# LINKAGE  1/ INDEPENDENT
ASSORTMENT.

42. CROSSING OVER
• The process by which exchange of chromosomal
segment takes place.
OR
• Recombination of linked genes.
• The term was given by MORGAN & MOTTLE.
• JANSEN (1909) – Observed chiasmata during
meiosis I.
• MORGAN – Chiasmata lead to crossing over by
breakage & reunion of homologous chromosomes.
• Unit of crossing over is CENTI MORGAN (CM)
given by – HALDANE.

43. • To explain the relationship between crossing over and chiasma following
theories were given –
CHIASMA
TYPE THEORY
• JANSSEN (1909).
• Act of crossing over is
followed by chiasma
formation.
• Crossing over takes place at
pachytene stage.
• Chiasma appears at
diplotene stage.
CLASSICAL
THEORY
• SHARP (1934).
• Crossing over is the result of
chiasma formation.
• Chiasma organised at
pachytene.
• Crossing over at diplotene.

44. • Crossing over takes place b/w 2 non - sister chromatids of homologous
chromosome.

46. MECHANISM OF CROSSING OVERCOPYCHOICEHYPOTHESIS
PRECOCITYHYPOTHESIS
CROSSOVERVALUE
COINCIDENCE
• DARLINGTON.
• Pairing of
homologues
occurs to avoid
singleness of a
chromosome.
• Occurs due to
strain.
• 2 chromosomes
pair during spiral
coiling and where
the tension
develops, the
chromosome
breaks and
recombination
occurs.
• BELLING
(1928).
• Chromosomes
represent the
genes which are
formed by joining
of inter -
chromomeric
regions.
• Chromomeres are
formed first
thereby the
interconnecting
links.
• Further they join
with other
homologous
chromosome.
• Percentage of
crossing over
varies in different
material.
• Measure of
interference.
• Calculated as –
ACTUAL
NUMBER OF
DOUBLE CROSS
OVER
EXPECTED
NUMBER OF
DOUBLE CROSS
OVER.

48. FACTORS CONTROLLING FREQUENCY
OF CROSSING OVER
1. Temperature
2. X – Ray
3. Age
4. Chemicals
5. Sex
6. Chiasmata formation
7. Inversions
8. Distance
9. Nutritional effect
10.Genotypic effect
11.Interference
12.Chromosome
structure effect
13.Centromere effect
# SIGNIFICANCE OF
CROSSING OVER –
1. Gives evidence that
genes are linearly +nt.
2. Provides operational
gene definition –
smallest heritable
segment of a
chromosome in the
interior of which no
crossing over takes
place.
3. Helpful in
chromosomal
mapping.
4. Main cause of genetic
variation.
5. Helpful in breeding
and evolving new
species.

49. CHROMOSOMAL MAPS/ LINKAGE MAP/
GENE MAP
• A linkage or genetic chromosome map is
a linear graphic representation of the
sequence & relative distances of the
various genes +nt in a chromosome.
# RECOMBINATION FREQUENCY –
• Frequency of crossing over is dependent
upon distance present b/w the two genes.
• If we say that 2 genes A & B are 10 map
units apart.
OR
• We can say that recombination frequency
is 10%.
Eg: We are saying genes A & B are 20
units apart & genes A & C are 10 units
apart. What is are the locations of gene A,
B & C?
#USES OF CHROMOSOMAL MAPS –
1. Find out the gene location.
2. Knowing recombination of various
genes.
3. Result prediction of di & tri hybrid
cross.

50. CHROMOSOMES
• Also k/as “HEREDITARY MATERIAL”.
• Responsible for genes transmission.
#DISCOVERY OF CHROMOSOMES –
• HOFMEISTER (1848) – first observed chromosomes in microsporocytes of
Tradescantia.
• ROUX (1883) – chromosomes take part in inheritance.
• W. WALDEYER (1888) – coined the term chromosome.
• WALTHER FLEMMING (1879) – splitting of chromosomes during cell division.
• BENDEN & BOVERI (1887) – found fixed number of chromosome in species.

51. KINDS OF CHROMOSOMES
VIRAL
In viruses,
bacteriophages, single
molecule of DNA or
RNA represents viral
chromosome.
PROKARYOTIC/
BACTERIAL/
NUCLEOID
In bacteria,
cyanobacteria.
Single large circular
DNA.
EUKARYOTIC
Individual specific
bodies formed due to
Deoxyribo -
nucleoprotein.

52. STRUCTURE OF CHROMOSOME
1. PELLICLE – Outer most chromosomal sheath.
2. MATRIX/ GROUND SUBSTANCE – Composed of
proteins, small quantities of RNA and lipid.
3. CHROMONEMATA – Coiled threads. Term given by
VEJDOVSKY (1912).
4. PRIMARY CONSTRICTION – Centromere.
5. SECONDARY CONSTRICTION/ NUCLEOLAR
ORGANIZER – Constriction other than primary. May be
+nt on one or both the arms. It is associated with
nucleolus formation during interphase. It contains genes
coding for 18S and 28S – NUCLEOLAR ORGANIZER
REGION (NOR).
6. CHROMOMERES – described by J. BELLINGS.
Chromonema thread at intervals is marked in linear order
by no. of knot or granules as in bead on string
appearance. Clear structures. Their no. is constant.

53. 7. TELOMERES – Tips of chromosome
which are rounded and sealed. It
prevents the ends of chromosome from
sticking to each other.
8. SATELLITE – Exhibited by some
chromosomes. It is the terminal part of
the chromosome beyond secondary
constriction. Elongated, rounded
variable in size. Chromosome bearing a
satellite – “SAT CHROMOSOME”.
9. CHROMATIDS – One copy of newly
formed chromosome which is still joined
to the original chromosome by a single
centromere. Two identical copies of
DNA are called as chromatids.

54. BASED UPON NO. OF
CENTROMERES
MONOCENTRIC
DICENTRIC
POLYCENTRIC
ACENTRIC
DIFFUSED/ NON
LOCATED
CLASSIFICATION OF CHROMOSOMES
CLASSIFICATION BASED UPON
CENTROMERE LOCATION
TELOCENTRIC
ACROCENTRIC
SUBMETACENTRIC
METACENTRIC

56. MOLECULAR ORGANIZATION OF
CHROMOSOME
• Based upon relative DNA
position & proteins positions in
the chromosomes.
CHROMOSOME
MULTIPLE
STRAND MODEL
STEFFENSEN 1952, RIS 1960 et
all.
Many DNA protein fibrils are +nt
in chromosome & at least 2
chromatids form a chromosome.
SINGLE STRAND
MODEL
TAYLOR, DUPROW
etc.
Chromosome is made
up of a single DNA
protein fibril.
SINGLE STRAND MODEL
FOLDED FIBRE MODEL
NUCLEOSOME MODEL
SOLENOID MODEL
DANGLER – STRING OR RADIAL
LOOP MODEL

57. SPECIAL TYPE OF CHROMOSOMES
1. POLYTENE CHROMOSOME – KOLLAR (1882) –
Described it and BALBIANI (1881) – Reported
Formed due to rapid division in chromosome without
cytoplasmic division (ENDOMITOSIS) due to which chromatids
don’t separate and remains attached to chromocenter.
It was first reported in salivary gland cells of insect.
2. LAMPBRUSH CHROMOSOME – FLEMMING WALTER
(1882).
Structure described by RUCKERT (1892).
Found in growing oocytes of several species excluding
mammals.
Has double main axis due to long chromatids.

58. • FLEMMING (1880) – Sustainable material present in nuclei is
chromatin.
• Largest chromosome is chromosome 1
and the smallest is chromosome Y.
CHROMATIN
HETEROCHROMATIN
Static chromatin.
Highly condensed
Transcriptionally inactive
EUCHROMATIN
Dynamic chromatin
Extended and open
Transcriptionally active

59. GENES
• Term given by JOHANNSEN (1909).
• T.H MORGAN (1910) defined gene as
“Any particle on the chromosome
which can be separated from other
particles by mutation or recombination
is called as a gene”.
• KHORANA awarded Nobel prize fro
the synthesis of artificial gene.
#GENE ACTION –
• Gene act by producing enzymes.
• Each gene synthesizes a particular
protein which acts as an enzyme &
brings about an appropriate change.
#MOLECULAR STRUCTURE –
• CISTRON/ FUNCTIONAL – BENZER
(1995) Cistron is that particular length
of DNA which is capable of producing
a protein molecule or polypeptide
chain or enzyme molecule.
• MUTON/ UNIT OF MUTATION – That
part of DNA which is capable of
undergoing mutation.
• RECON – That length of DNA which
is capable of undergoing
recombination.

60. SOME SPECIFIC TERMS
• TRANSPOSONS OR JUMPING
GENES – HEDGES & JACOB
(1974) gave the term and reported
this in bacteria. Those DNA
segments which can join with other
DNA segments completely
unrelated & thus forming an
illegitimate pairing.
• RETROPOSONS – ROGERS
(1983). Those DNA segments
which are formed from RNA or
from reverse transcription.
• SPLIT GENES OR INTERRUPTED
GENES – R. ROBERTS & P.
SHARP reported this in
mammalian virus & then in
eukaryotes. These genes break up
into pieces/ segments –
• EXONS = Coding segment.
• INTRONS = Non coding segments.
• SPLIT GENES = EXONS +
INTRONS.
• FALSE GENES/ PSEUDOGENES
– Copy of cistron which is
defective.

61. SEX DETERMINATION
• MALE HETEROGAMETY – Male forms
two types of gametes.
• FEMALE HETEROGANETY – Female
forms two types of gametes.
SEX DETERMINATION IN GRASSHOPPER
SEXDETERMINATIONIN
HUMANBEINGS
SEX DETERMINATION IN BIRDS

62. CRISS – CROSS INHERITANCE
• Morgan’s case of eye colour inheritance in
Drosophila is easily explained by assuming
that gene for pigment in eye is carried on X
chromosome and that the Y chromosome
has no allele of this gene.
• Male transmits its X chromosome t his
daughters only, while the females to both
the sons and daughters.
• The transmission of a gene from mother to
son or father to daughter is CRISS –
CROSS INHERITANCE.
CRISS -
CROSS
DIGYNIC
P (Male) –
F1(Female) –
F2(Male)
DIANDRIC
P (Female) –
F1(Male) –
F2(Female)

63. PEDIGREE ANALYSIS
• A record of the inheritance of a trait in several generations of a human family
is called PEDIGREE ANALYSIS.
• 1. AUTOSOMAL DOMINANT
• This trait never skips a generation
• & marriage b/w a normal & affected
• Individual leads to 1:1 ratio
• of normal to affected child.
• 2. AUTOSOMAL
• RECESSIVE – (Sickle cell
• anaemia)
• 3. X – LINKED DOMINANCE
• 4. X – LINKED RECESSIVE
• 5. Y - LINKED
PROBAND
Person from whom the case
history of pedigree starts.
PROPOSITY
Male
PROPOSITA
Female

64. DISORDERS
• MANDALIAN – Alteration or mutation in the single gene.
• CHROMOSOMAL – Absence, excess or abnormality of one or more
chromosomes.
• Haemophilia
• Colour blindness
• Sickle cell anaemia
• Phenylketonuria
• Cystic fibrosis
MANDALIAN
DISORDERS
• Down’s syndrome/ Mongolism
• Patau’s syndrome
• Cri-du-chat syndrome
• Klinefelter's syndrome
CHROMOSOMAL
DISORDERS

65. HAEMOPHILIA/ BLEEDER’S DISEASE
• Sex linked recessive disorder.
Shows it transmission from
unaffected carrier female to some
of the male progeny.
• Inherited through X linked
recessive gene.
• Expressed in males carried
through females.

66. COLOUR
BLINDNESS
SICKLE CELL
ANAEMIA
PHENYLKETONURIA
• Autosomal recessive trait.
• Individual lacks an enzyme which is
responsible for converting amino acid
phenylalanine into tyrosine.

67. CHROMOSOMAL DISORDERS
• ANEUPLOIDY – Gain/loss of
chromosome.
• POLYPLOIDY – Increase in
chromosome set.
•
• TRISOMY 21.
• First described by LANGDON DOWN
(1866).
• Short statured, small round head,
furrowed tongue, partially open
mouth.
• Palm is broad with characteristic
palm crease.
• Physical, psychomotor and mental
development retarted.
DOWN’S SYNDROME

68. • Additional X chromosome in males.
• XXY – Karyotype.
• 47 chromosomes.
• Sterile males.
• Absence of X chromosome in females.
• X0 – Karyotype.
• 45 chromosomes.
• Sterile ovaries.
KLINEFELTER’S SYNDROME
TURNER’S SYNDROME