Presentation for grade 12, CBSE Students

1. Principles of Inheritance and
Variation

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.

5. 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

6. • Genetics is the branch of life science that deals with the
study of heredity and variation.
• Heredity is 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.
Terminology

7. Gregor Johann Mendel: Father of Genetics
• 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 the significance of his
work was not recognized during
his lifetime, it has become the
basis for the present-day field of
genetics.

8. • 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
Mendel’s ApproAch

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

10. • 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.
Terminologies

11. • 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.

12. 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.

13. 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

15. • 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)

16. TallP Dwarfx
F1
All Tall
Phenotype
Monohybrid cross
F2
Tall is dominant
to Dwarf
TT tt
Tt
Genotype
Homozygous Dominant Homozygous Recessive
HeterozygousSelf pollinated
Gamets T t
T TT
tall
Tt
tall
t Tt
Tall
tt
dwarf
Phenotypic ratio 3:1 Genotypic ratio: 1:2:1

17. • 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

18. 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.

19. 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 of 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

20. 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

21. 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

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

24. 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

25. 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.

26. • I
A and I
B 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

28. • 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 having
two 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

29. 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.

30. Dihybrid cross:
Parents
Round Yellow Wrinkled Green
Genotype
Phenotype
RRYY rryy
Gametes RY ry
F1 generation
Round Yellow
RrYy

31. Phenotypic ratio : 9 : 3 : 3 : 1

32. 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

33. • 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

34. 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.

35. Chromosomal Theory of inheritance

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

37. • 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.

38. 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.

39. 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)

40. 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.

41. 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.

42. 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

43. • 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

45. • 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.

46. 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.

47. 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.

48. 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

49. Hemophilia:
• 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

52. H

53. Sickle cell anemia

54. • 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

56. 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

58. 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

59. 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

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

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