Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms. Though heredity had been observed for millennia, Gregor Mendel, Moravian scientist and Augustinian friar working in the 19th century in Brno, was the first to study genetics scientifically. Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring over time. He observed that organisms (pea plants) inherit traits by way of discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene. Trait inheritance and molecular inheritance mechanisms of genes are still primary principles of genetics in the 21st century, but modern genetics has expanded beyond inheritance to studying the function and behavior of genes. Gene structure and function, variation, and distribution are studied within the context of the cell, the organism (e.g. dominance), and within the context of a population. In science and especially in mathematical studies, a variational principle is one that enables a problem to be solved using calculus of variations, which concerns finding functions that optimize the values of quantities that depend on those functions.

1. PRINCIPLES OF
INHERITANCE & VARIATION
AJAY KUMAR GAUTAM
CHAPTER - 5

2. Mendelian Genetics &Variation
All living organisms reproduce.
It results in the formation of offspring of the same kind.
The resulting offspring most often do not totally resemble the parent.
Siblings sometimes look so similar to each other or sometimes even so
different.

3. Mendelian Genetics &Variation
Genetics : It is a branch of biology which deals with the inheritance, as
well as the variation of characters from parents to offspring.
Inheritance : Inheritance is the process by which characters are passed on
from parent to progeny; it is the basis of heredity.
Heredity : Heredity is the transmission of characters from parents to their
offsprings.
Variation :Variation is the degree by which progeny differ from their
parents.
Environmental variation:These are aquired and non heritable.
Heriditary variations :These are genetical and inheritable.

4. Gregor Johann Mendel
• He is known as the ‘Father of Genetics‘.
• Gregor Mendel, conducted hybridisation experiments on garden peas for
seven years (1856-1863).
• Proposed the laws of inheritance in living organisms.
• During Mendel’s investigations into inheritance patterns it was for the first
time that statistical analysis and mathematical logic were applied to problems
in biology.
• Mendel investigated characters in the garden pea plant that were manifested
as two opposing traits, e.g., tall or dwarf plants, yellow or green seeds.
• Mendel selected 14 true-breeding pea plant varieties, as pairs which were
similar except for one character with contrasting traits. Some of the
contrasting traits selected were smooth or wrinkled seeds, yellow or green
seeds, inflated (full) or constricted green or yellow pods and tall or dwarf
plants.

5. Seven pairs of contrasting traits in
pea plant studied by Mendel

6. 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.

7. 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.

8. 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.

9. 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.

11. 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 heterozygoteTt of F1 was same as parent withTT & proposed, in a
pair of dissimilar factors one dominates the other & hence called dominant (T) & recessive (t).

12. MONOHYBRID CROSS
P Phenotype Tall Dwarf
Genotype Tt tt
Homozygous Dominant Homozygous Recessive
F1 Tt (Tall is dominant to Dwarf)
AllTall (Hetrozygous)
Self Pollination
GAMETES T t
T TT
Tall
Tt
Tall
t Tt
Tall
tt
Short
Phenotypic Ratio =
3:1
Genotypic Ratio =
1:2:1

13. Punnett Square
Production of gametes & formation of zygotes.
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.
Phenotypic Ratio =Tall : Dwarf
3:1
Genotypic Ratio =TT :Tt : tt
1:2:1

14. TEST CROSS
• Test cross is a cross between an organism with unknown genotype and a
recessive parent. It is used to determine whether an individual is homozygous
or heterozygous for a trait.

15. MENDELIAN LAW OF INHERITANCE
• Rules were proposed by Mendel as Law or
Principles of Inheritance :
i. Law of Dominance
ii. Law of Segregation
iii. Law of IndependentAssortment

16. Law of Dominance
Characters are controlled by discrete units called Factors
Factors occurs in pair
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

17. 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.

18. 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 cros
• Ex snapdragon. ( Dog flower plant)

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

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

22. 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 I 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.

23. IA and I are co-dominant and dominant over i.

25. • 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

26. PRINCIPLES OF
INHERITANCE & VARIATION
AJAY KUMAR GAUTAM
CHAPTER - 5

27. Inheritance ofTwo 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.

28. Dihybrid Cross
Parents Phenotype RoundYellow Wrinkled Green
Genotype RRYY rryy
Gametes RY ry
F1 generation RrYy
RoundYellow

29. Phenotypic ratio : 9 : 3 : 3 : 1

30. Phenotypic ratio – 1 : 1 : 1 :1
• 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
DihybridTest Cross

31. • 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 accepted .
4. No proof for existence of factors.
DihybridTest Cross

32. Chromosomal Theory of Inheritance
• It was proposed byWalter Sutton andTheodore 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.

33. Chromosomal Theory of Inheritance

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

35. • 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
Chromosomal Theory of Inheritance

36. 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.

37. XX-XOType
 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 + XO)

38. XX-XYType
•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.

39. ZZ – ZWType
• 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.

40. 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.

41. 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.

42. Genetic Disorder
• 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

43. Colour Blidness
1. It is a sex-linked recessive disorder due to defect in either red or green cone of eye
resulting in failure to discriminate between red and green colour.
2. This defect is due to mutation in certain genes present in the X chromosome.
3. It occurs in about 8 per cent of males and only about 0.4 per cent of females.
4. This is because the genes that lead to red-green colour blindness are on the X
chromosome
5. Males have only one X chromosome and females have two
6. The son of a woman who carries 2022-23 90 BIOLOGY the gene has a 50 per cent
chance of being colour blind
7. The son of a woman who carries 2022-23 90 BIOLOGY the gene has a 50 per cent
chance of being colour blind
8. Its effect is suppressed by her matching dominant normal gene

44. 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

45. Hemophilia
FATHER WITH
HEMOPHILIA AND
MOTHER WHO IS NOT A
CARRIER

46. Hemophilia
CARRIER MOTHER AND
FATHER WITHOUT
HEMOPHILIA

47. Hemophilia

48. SICKLE CELL ANEMIA
• This is an autosome linked recessive trait that can be transmitted from
parents to the offspring when both the partners are carrier for the
gene (or heterozygous).
• The disease is controlled by a single pair of allele, HbA and HbS ,Out
of the three possible genotypes only homozygous individuals for
HbS (HbSHbS ) show the diseased phenotype.
• Heterozygous (HbAHbS ) individuals appear apparently unaffected
but they are carrier of the disease as there is 50 per cent probability of
transmission of the mutant gene to the progeny, thus exhibiting
sickle-cell trait.
• The defect is caused by the substitution of Glutamic acid (Glu) by
Valine (Val) at the sixth position of the beta globin chain of the
haemoglobin molecule.

49. • The substitution of amino acid in
the globin protein results due to
the single base substitution at the
sixth codon of the beta globin
gene from GAG to GUG.
• The mutant haemoglobin
molecule undergoes
polymerisation under low oxygen
tension causing the change in the
shape of the RBC from biconcave
disc to elongated sickle like
structure.
SICKLE CELL ANEMIA

50. • The disease. Sickle-cell anaemia :This is an autosome linked
recessive trait that can be transmitted from parents to the
offspring when both the partners are carrier for the gene (or
heterozygous)
• The disease is controlled by a single pair of allele, HbA and HbS .
• Out of the three possible genotypes only homozygous individuals
for HbS (HbSHbS ) show the diseased phenotype
• Heterozygous (HbAHbS ) individuals appear apparently
unaffected but they are carrier of the disease as there is 50 per
cent probability of transmission of the mutant gene to the
progeny,
SICKLE CELL ANEMIA

51. • Thus exhibiting sickle-cell trait .The defect is caused by the substitution of Glutamic
acid (Glu) byValine (Val) at the sixth position of the beta globin chain of the
haemoglobin molecule
• The substitution of amino acid in the globin protein results due to the single base
substitution at the sixth codon of the beta globin gene from GAG to GUG.
• The mutant haemoglobin molecule undergoes polymerisation under low oxygen
tension causing the change in the shape of the RBC from biconcave disc to
elongated sickle like structure
SICKLE CELL ANEMIA

52. Phenylketonuria
• This inborn error of metabolism is also inherited as the autosomal recessive
trait.
• The affected individual lacks an enzyme that converts the amino acid
phenylalanine into tyrosine.
• Phenylalanine is accumulated and converted into phenylpyruvic acid and other
derivatives. Accumulation of these in brain results in mental retardation.
• These are also excreted through urine because of its poor absorption by kidney.

53. Thalassemia
• This is also an autosome-linked recessive
blood disease transmitted from parents to
the offspring when both the partners are
unaffected carrier for the gene (or
heterozygous).
• The defect could be due to either mutation
or deletion which ultimately results in
reduced rate of synthesis of one of the
globin chains (α and β chains) that make up
haemoglobin.
• This causes the formation of abnormal
haemoglobin molecules resulting into
anaemia which is characteristic of the
disease.

54. • Thalassemia can be classified according to which chain of the haemoglobin
molecule is affected. In αThalassemia, production of α globin chain is affected
while in βThalassemia, production of β globin chain is affected.
• αThalassemia is controlled by two closely linked genes HBA1 and HBA2 on
chromosome 16 of each parent and it is observed due to mutation or deletion
of one or more of the four genes.
• The more genes affected, the less alpha globin molecules produced.While β
Thalassemia is controlled by a single gene HBB on chromosome 11 of each
parent and occurs due to mutation of one or both the genes.
• Thalassemia differs from sickle-cell anaemia in that the former is a
quantitative problem of synthesising too few globin molecules while the latter
is a qualitative problem of synthesising an incorrectly functioning globin.
Thalassemia

55. Chromosomal Disorders
• The chromosomal disorders on the other hand are caused due to absence
or excess or abnormal arrangement of one or more chromosomes.
• Failure of segregation of chromatids during cell division cycle results in
the gain or loss of a chromosome(s), called aneuploidy
• Turner’s syndrome results due to loss of an X chromosome in human
females
• Failure of cytokinesis after telophase stage of cell division results in an
increase in a whole set of chromosomes in an organism and, this
phenomenon is known as polyploidy.

56. • The total number of chromosomes in a normal human cell is 46
(23 pairs).
• Out of these 22 pairs are autosomes and one pair of
chromosomes are sex chromosome.
• Sometimes, though rarely, either an additional copy of a
chromosome may be included in an individual or an individual
may lack one of any one pair of chromosomes.These situations
are known as trisomy or monosomy of a chromosome
• Down’s syndrome,Turner’s syndrome, Klinefelter’s syndrome
are common examples of chromosomal disorders
Chromosomal Disorders

57. Down’s Syndrome
• The cause of this genetic disorder
is the presence of an additional
copy of the chromosome number
21 (trisomy of 21).
• This disorder was first described
by Langdon Down (1866).
• The affected individual is short
statured with small round head,
furrowed tongue and partially
open mouth (Figure 5.16).
• Palm is broad with characteristic
palm crease. Physical,
psychomotor and mental
development is retarded.

58. Klinefelter’s Syndrome
• This genetic disorder is also caused due to the
presence of an additional copy of
Xchromosome resulting into a karyotype of
47, XXY.
• Such an individual has overall masculine
development, however, the feminine
development (development of breast, i.e.,
Gynaecomastia) is also expressed.
• Such individuals are sterile. Klinefelter’s Syndrome
Symptoms

59. Turner’s Syndrome
• Such a disorder is caused due to the absence of one of the X
chromosomes, i.e., 45 with X0
• Such females are sterile as ovaries are rudimentary besides
other features including lack of other secondary sexual
characters