This document provides information about principles of inheritance and variation in genetics. It discusses key topics including: - Genetics deals with inheritance and variation from parents to offspring. Variation results in offspring differing from parents. - Gregor Mendel conducted experiments with pea plants in the 1800s and established the principles of heredity, including dominance, segregation, independent assortment. He demonstrated genes are passed from parents to offspring in predictable ratios. - Chromosomal theory of inheritance later explained that genes are located on chromosomes and segregate during gamete formation according to Mendel's laws. The work of Morgan, Sutton, and Boveri supported this theory through experimentation.

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