PRINCIPLES OF IMHERITANCE AND VARIATION

1. RALLI INTERNATIONAL SCHOOL

3. Genetics
• Organisms reproduce- formation of
offspring of the same kind.
• The resulting offspring most often do not
totally resemble the parent.
• Branch of biology that deals with the
inheritance and variation- 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.

4. History • Human knew before
8000- 1000 B. C
variation is due to
sexual reproduction
• Exploited variations
present in wild plants
& animals to
selectively breed &
select organism with
desirable characters
• Artificial selection &
domestication of wild
cow- Sahiwal cows in
Punjab

5. Terminology
• Genetics is the branch of life science that deals with the
study of heredity and variation.
• Heredityis the transmission of characters from parents to
their offsprings.
• Variation is the difference among the offsprings and with
their parents.
• Hereditary variations: These are genetical and
inheritable.
• Environmental variation: These are acquired and
non inheritable.

6. Gregor Johann Mendel: Father ofGenetics
• Known as the father of
modern genetics
• Gregor Mendel developed
the principles of heredity
while studying seven
pairs of inherited
characteristics in pea plants.
• Although thesignificance of
his work was not recognized
during his lifetime, it has
become the basis for the
present-day field of genetics.

7. Mendel’s ApproAch
• Conducted hybridization (artificial pollination/ cross
pollination) experiment for 7 years 1856-1863 & proposed
law of inheritance
• Applied statistical analysis & mathematical logic for
biology problems
• Large sampling size- greater credibility to data
• Experiments- true breeding pea lines
(continuous self pollination)
• Confirmation of inference from experiments on
successive generations of test plants, proved general
rules of inheritance
• Mendel investigated two opposing traits- tall & dwarf,
yellow & green seed

8. Seven pair of contrasting characters selected
by Mendel for his experiment.

9. Terminologies
• Phenotype: The external appearance of an organism due to
the influence of genes and environmental factors.
• Genotype: The genetic constitution of an individual
responsible for the phenotype .
• Phenotypic ratio: The correct proportion of phenotype
in population.
• Genotypic ratio: The correct proportion of genotype
in population.
• Homozygous: The individual heaving identical genes in
an allelic pair for a character. Ex: TT, tt.
• Heterozygous: The individual heaving un-identical genes
in an allelic pair for a character. Ex: Tt.

10. Terminologies
• Dominant gene: The gene that expresses its character in
heterozygous condition.
• Recessive: The gene that fails to express its character
in heterozygous condition.
• Hybrid: The progeny obtained by crossing two parents
that differ in characters.
• Back cross: The cross between F1 hybrid and one of
its parents.
• Test cross: The cross between hybrid and its
homozygous recessive parent. It is used to identify the
genotype of the hybrid.

11. Why Mendel selected pea plant?
• Pure variety are available.
• Pea plants are easy to cultivate.
• Life cycle of plants are only few months. So that
result can be got early.
• Contrasting trait are observed.
• Flowers are bisexual and normally self pollinated.
• Flowers can be cross pollinated only manually.
• Hybrids are fertile.

12. Inheritance of one gene
• Inheritance of one gene can be explained by monohybrid
cross.
• The cross between two parents differing in one pair of
contrasting character is called monohybrid cross.
• Crossed tall & dwarf pea plants- Collected seeds & grew to
generate first hybrid generation/ Filial generation/F1
• F1 plants- Tall & none were dwarf
• For other traits also- F1 generation resembled only one
parent & trait of other parent were not shown
• Self pollinated F1 –Filial 2 generation/ F2
• F2 generation- 1/4th were dwarf & 3/4th tall- identical to
parents
• F1 generation one parent trait shown & F2 both parent trait
shown in the ratio- 3:1 & no blending were seen

14. • 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
• Plants pair of alleles for height- TT, Tt & tt
• Mendel proposed- true breeding tall or dwarf plant- identical
or homozygous allele pair of TT or tt (genotype)
• Descriptive term tall or dwarf- phenotype
• 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)

15. P x
F
1 All
Tall
Monohybrid Cross
F
2
Tall is dominant
to Dwarf
Tal
l
T
T
Dwar
f
tt
Phenotyp
e
Genotyp
e
Homozygous
Dominant
Tt
Homozygous
Recessive
Heterozygo
us
Self
pollinated Gamets T t
T TT
tall
Tt
tall
t Tt
Tal
l
tt
dwar
fPhenotypic ratio Genotypic ratio:

16. • Production of gametes & formation of zygotes-
Punnett Square
• Developed by- British scientist Reginald C. Punnett
• Graphical representation- calculate probability of
possible genotypes in genetic cross
• Gametes- on two sides, top row & left columns
• Self- pollination- 50%
• F2- 3/4th tall & 1/4th
Dwarf- phenotypically
• 1/4 : ½ : ¼ ratio of TT:
Tt: tt- genotype

17. Test cross: The cross between hybrid
and its homozygous recessive parent I
called test cross. It is used to identify the
genotype of the hybrid.

18. Mendelian laws of heredity.
• Rules were proposed- Principles or Laws of
Inheritance: First Law or Law of Dominance &
Second law or Law of Segregation
• Law o f dominance
1. Characters are controlled by discrete units called
Factors
2. Factors occurs in pair
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

19. Law of segregation
• It states that, ‘when a pair of factors for a character
brought together in a hybrid, they segregate
(separate) during the formation of gametes.
• Alleles do not blend & both characters recovered in F2
& one in F1
• Factors which is present in parent segregate &
gametes receives only one of two factors
• Homozygous parent- one kind gamete
• Heterozygous parent- two kind gamete each one have
one allele with equal proportion

20. Incomplete dominance:
• Correns discovered Incomplete dominance in
Merabilis jalapa.
• It is also called partial dominance, semi dominance.
• The inheritance in which allele for a specific character
is not completely dominant over other allele is called
Incomplete dominance.
• 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 ratio same as Mendelian cross & Phenotype
ratio different than Mendelian cross

21. Incomplete dominance: Ex snapdragon.
( Dog flower plant)

23. Parent: Red X White
Genotype
.
RR WW
Gametes R W
F1 generation Pink (Hybrid)
RW
Self pollination
F2 generation
Gametes R W
R RR
Red
RW
Pink
W RW
Pink
WW
white
The phenotypic ratio is
1:2:1.
The genotypic ratio is
1:2:1

24. 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.
• The ABO locus is located on chromosome 9.
• IA is responsible for production of antigen –A.
• IB is responsible for production of antigen –B.
• i does not produces any antigen.

25. • IA and IB are co-dominant and dominant
over i.
Blood Group Genotype
A- Group IAIA or IA i
B-Group IBIB or IBi
AB-Group IAIB
O-Group ii

26. Genetic Basis of Blood Groups in Human Population

27. • ABO blood grouping- multiple allele
• Three alleles govern same character
• Multiple allele is found when population studies are
made
• Single gene product may produce more than one effect
• Eg.- Starch Synthesis in Pea seeds- controlled by a gene
havingtwo allele B & b
• Starch synthesis effective if homozygote BB & produce
large starch grains
• Homozygote bb –lesser efficiency in starch synthesis &
seeds are wrinkled
• Heterozygote Bb –round seeds, intermediate size

28. Inheritance of two gene:
Mendel’s 2nd law or Law of independent
assortment:
• It states that, ‘factors for different pairs of contrasting
characters in a hybrid assorted (distributed)
independently during gamete formation.
Mendel’s 2nd law can be explained by dihybrid cross.
• Dihybrid cross: The cross between two parents, which
differs in two pairs of contrasting characters.

29. Dihybrid cross:
Round
Yellow
Wrinkled
Green
Genotyp
e
Parents
Phenotyp
e
Gamete
s
RRY
Y
RY
rry
y
ry
F1
generation
RrYy
Round
Yellow

30. Phenotypic ratio : 9 : 3 : 3
: 1

31. Dihybrid test cross.
• F1 hybrid is crossed with recessive green wrinkled pea
plant.
• Recessive green wrinkled – rryy, Gamete ry
• F1 hybrid : round yellow- RrYy, Gametes:
RY, Ry, rY, ry.
Gametes RY Ry rY ry
ry RrYy Rryy rryY rryy
Phenotypic ratio – 1 : 1 : 1 :1

32. • Mendel work published 0n 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 acceted
4. No proof for existence of factors

33. Chromosomal theory of inheritance:
• It was proposed by Walter Sutton and Theodore Boveri .
• They work out the chromosome movement during meiosis.
• The movement behavior of chromosomes was parallel to the
behavior of genes. The chromosome movement is used to
explain Mendel’s laws.
• The knowledge of chromosomal segregation with
Mendelian principles is called chromosomal theory of
inheritance.
• According to this, Chromosome and genes are present in
pairs in diploid cells.
• Homologous chromosomes separate during gamete
formation (meiosis)
• Fertilization restores the chromosome number to diploid
condition.

34. Chromosomal Theory of inheritance

35. • Thomas Hunt Morgan and his colleagues conducted
experimental verification of chromosomal theory of
inheritance
• Morgan worked with tiny fruit flies, Drosophila
melanogaster.

36. • He selected Drosophila because,
• It is suitable for genetic studies.
• Grown on simple synthetic medium in the laboratory.
• They complete their life cycle in about two weeks.
• A single mating could produce a large number of progeny
flies.
• Clear differentiation of male and female flies
• Many types of hereditary variations can be seen with low
power microscopes.

37. SEX DETERMINATION
• Henking (1891) traced specific nuclear structure
during spermatogenesis of some insects.
• 50 % of the sperm received these specific
structures, whereas 50% sperm did not
receive it.
• He gave a name to this structure as the X-body.
• This was later on named as X-chromosome.

38. XX-XO type
• Sex-determination of grass hopper:
• The grasshopper contains 12 pairs or 24
chromosomes. The male has only 23 chromosome.
• All egg bears one ‘X’ chromosome along with
autosomes.
• Some sperms (50%) bear’s one ‘X’ chromosome and
50% do not.
• Egg fertilized with sperm having ‘X’ chromosome
became female (22+XX).
• Egg fertilized with sperm without ‘X’ chromosome
became male (22 + X0)

39. XX-XY type
Sex determination in insects and
mammals
• In this type both male and female has same
number of chromosomes.
• Female has autosomes and a pair of X chromosomes.
(AA+ XX)
• Male has autosomes and one large ‘X’ chromosome and
one very small ‘Y-chromosomes. (AA+XY)
• In this type male is heterogamety and female
homogamety.

40. ZZ – ZW type
Sex determination in
birds:
• In this type female birds has two different sex chromosomes
named as Z and W.
• Male birds have two similar sex chromosomes and called ZZ.
• In this type of sex determination female is heterogamety and
male is homogamety.

41. Linkage recombination
• Morgan carried dihybrid cross in Drosophila to study
genes that are sex linked
• Crossed- yellow bodied, white eyed females with
brown bodied, red eyed males & intercourse F1
progeny
• Two genes did not segregate independently of each other
& F2 ratio deviated from 9:3:3:1
• The genes present on X –chromosome & two genes in a
dihybrid cross- situated on same chromosome- parental
gene combination is much higher than non
parental- this is due to physical association/ linkage of
two genes on chromosome- Linkage
• Generation of non parental combination-
Recombination

42. • He found genes are grouped in same chromosome, some
genes
are tightly linked- less recombination
• When genes are present in different chromosome-
higher recombination
• Eg.- Genes for white & yellow- tightly linked- 1.3%
recombination while genes for white & miniature
wings- 37.2% recombination
• Student Alferd Sturtevant used frequency of
recombination between gene pairs on chromosome as
a measure of the distance between genes & mapped
genes position on chromosome

44. • Linkage: physical association of genes on a chromosome is
called linage.
• Recombination: The generation of non-parental
gene combinations is called recombination.
• It occurs in crossing over of chromosomes during meiosis.

45. MUTATION
• Phenotypic variation occurs due to change in gene or
DNA sequence is called mutation. The organism that
undergoes mutation is mutant.
• Phenomenon which result in alternation of DNA
sequence & result in change in genotype & phenotype
1. Loss (deletion) or gain (insertion/duplication) of a
segment of DNA results in alteration in chromosomes-
abnormalities/ aberrations- Chromosomal
aberrations
2. Gene Mutations:The mutation takes place due to change
in a single base pair of DNA is called gene mutation or
point mutation. E.g. sickle cell anemia.
3. Frame shift mutations: Deletion or insertions of base
pairs of DNA is called frame shift mutations.

46. Pedigree
Analysis:
• The study of inheritance of
genetic traits in several
generations of a family is
called the pedigree analysis.
• Pedigree study- strong tool
of human genetics to trace
inheritance of specific
trait/ abnormality/
diseases
• Pedigree analysis of
inheritance of a traits is
represented in family tree
• It helps in genetic counseling
to avoid genetic disorders.

47. Genetic disorders
• Genetic disorders grouped into two categories –
1. Mendelian disorder
2. Chromosomal disorder
Mendelian Disorders
• Mendelian disorders are mainly determined by alteration or
mutation in the single gene.
• It obey the principle of Mendelian inheritance (principles of
inheritance) during transmission from one generation to other.
• Mendelian disorder- traced in family by pedigree analysis
• E.g. Haemophilia, Colorblindness, Cystic fibrosis, Sickle
cell anemia, Phenylketonuria, Thalesemia etc.
• Dominant or recessive- pedigree analysis
• Trait may linked to sex chromosome, Eg. Haemophilia
• X- linked recessive trait- transmitted from carrier female to
male progeny

48. Haemophilia:
• It is a sex linked recessive disease.
• The defective individual continuously bleed to a simple cut.
• The gene for hemophilia is located on X chromosome.
• In this disease a single protein that is a part of cascade of
proteins
that involved in the clotting of blood is affected.
• The diseases transmitted from unaffected carrier female to
some of the male progeny.
• Heterozygous female (carrier)- transmit to sons
• Female being hemophilic is rare- Mother should be
carrier & father Haemophilic

51. H

52. Sickle cell anemia

53. • Autosome linked recessive trait
• Transmitted from parents- both partners are
carrier/ heterozygous
• Controlled by single pair of allele HbA & Hbs
• Homozygous individuals of Hbs (HbSHbS)- diseased
• Heterozygous individuals HbAHbS- unaffected but carrier
• Defect is due to substitution of Glutamic acid(Glu) by
Valine (Val)- at the 6th position beta globin chain of Hb
• Due to substitution of single base at 6th codon of beta
globin gene from GAG to GUG
• Mutant haemoglobin- polymerization under low oxygen
tension causing change in shape of RBC from biconcave
to sickle like structure

55. Phenylketonuria
• Inborn error of metabolism- inherited as
autosomal recessive trait
• Affected individual lack enzyme that convert amino
acid phenylalanine to tyrosine
• Phenylalanine accumulates & convert to
phenylpyruvic acid & other derivatives
• Accumulation in brain result- mental retardation
• Excreted in urine- poor absorption by kidney

56. Chromosomal Disorder
• Caused due to absence or excess or abnormal
arrangement of one or more chromosome.
Causes:
1. Failure of segregation of chromatids- cell division cycle-
gain or loss chromosome- Aneuploidy, Eg.- Down’s
syndrome (Extra copy of 21 chromosome)- Trisomy,
Turner’s syndrome (loss of an X chromosome in female)-
Monosomy
2. Failure of cytokinesis after telophase- increase in
whole set chromosomes- Polyploidy, seen in plants

57. Down's Syndromes
• Presence of an additional copy of chromosome no. 21-
Trisomy of 21
• Described- Langdon Down (1866)
• Short statured, small round head, furrowed tongue,
partially open mouth, broad palm with palm crease;
physical, psychomotor & mental- retardation

58. Klinefelter’s Syndrome
• Presence ofan additional copy of X- chromosome
• Karyotype- 47, XXY
• Overall masculine development along with feminine
development- development of breast (Gyanaecomastia),
Sterile

59. Turner’s Syndrome
• Absence of one of X- chromosome, Monosomy
• Karyotype- 45, X0
• Females-sterile, ovaries are rudimentary, lack of
secondary sexual character