This document provides definitions and explanations of key genetics concepts and terminology. It begins by defining genetics as the science of heredity and the transmission of traits from parents to offspring. It then discusses DNA and genes, chromosomes, alleles, phenotypes, and other basic genetic concepts. It also covers different types of mutations and how they can cause genetic disorders. Finally, it classifies genetic diseases and describes patterns of inheritance for single-gene disorders like autosomal dominant, autosomal recessive, and X-linked recessive traits.

1. GeneticsGenetics

2. MeaningMeaning
 The term Genetics was introduced by
Bateson in 1906. It was derived from
Greek word “Gene” which means ‘to
become’ or ‘to grow into’.
 Therefore, genetics is the science of
coming into being.

3. WHAT IS GENETICS?????WHAT IS GENETICS?????
 The branch of biology that deals with
heredity, especially the mechanisms of
hereditary transmission and the
variation of inherited
characteristics among
similar or related
organisms

4.  “ Genetics is that branch of
biological sciences which deals
with the transmission of
characteristics from parent to off
spring.”

5. Terminology:Terminology:

6. What Is DNA?
 DNA (deoxyribonucleic acid) carries the
genetic information in the body’s cells.
 DNA is made up of four similar chemicals
(called bases and abbreviated A, T, C, and
G) that are repeated over and over in pairs.
 Adenosine, Thymine, Cytosine, Guanine

7. What Is a Gene?
 A gene is a distinct portion of a cell’s DNA.
 Genes are coded instructions for making
everything the body needs, especially proteins.
 Human beings have about 25,000 genes.
Researchers have discovered what some of our
genes do, and have found some that are associated
with disorders (such as cystic fibrosis or
Huntington’s disease). There are, though, many
genes whose functions are still unknown.

8. GENEGENE
 Gene hold the information to build and
maintain their cells and pass genetic traits to
off springs

9. ALLELEALLELE
 alleles are pairs or series of genes on a
chromosome that determine the hereditary
characteristics.. An example of an allele is
the gene that determines hair color.

10.  A dominant allele produces a
dominant phenotype in
individuals who have one copy of
the allele, which can come from
just one parent.
 For a recessive allele to produce a
recessive phenotype, the
individual must have two copies,
one from each parent.
 An individual with one dominant
and one recessive allele for a
gene will have the dominant
phenotype. They are generally
considered “carriers” of the
recessive allele: the recessive
allele is there, but the recessive
phenotype is not.

11.  A phenotype (from Greek phainein,
meaning "to show", and typos, meaning
"type") is the composite of an organism's
observable characteristics or traits, such as
its morphology, development, biochemical
or physiological properties, phenology,
behavior, and products of behavior (such as
a bird's nest).

14.  Homozygous-an organism in which 2 copies of
genes are identical i.e. have same alleles
 Homozygous means you carry two genes that are
the same for each trait. Example: BB for brown
eyes.
 Heterozygous-an organism which has different
alleles of the gene or Heterozygous means you carry
two different genes for each trait Heterozygous-an
organism which has different alleles of the gene
 Example: Bb would also be brown eyes, though the
small "b" would be the gene for blue eyes. It
doesn't show up in the phenotype (outward
appearance) because the gene for brown eyes is
dominant. If someone had blue eyes they would be
homozygous "bb".

15. Homozygous and HeterozygousHomozygous and Heterozygous
 Here BB & bb is the homozygous and Bb is
the heterozygous

17. What Are Chromosomes?
 Genes are packaged in bundles
called chromosomes.
 Humans have 23 pairs of
chromosomes (for a total of 46).
 Of those, 1 pair is the sex
chromosomes (determines
whether you are male or female),
and
 the other 22 pairs are autosomal
chromosomes (determine the
rest of the body’s makeup).

18. Chromosomes:
 Chromatin: made up of DNA, RNA &
proteins that make up chromosome,
Chromatin is located in the
nucleus of a cell. During
prophase of mitosis, chromatin
fibers become coiled into
chromosomes
 Chromatids: one of the two identical
parts of the chromosome.
 Centromere: the point where two
chromatids attach

20.  NUCLEOTIDE: group of molecules that when
linked together, form the building blocks of DNA
and RNA; composed of phosphate group, the
bases: adenosine, cytosine, guanine and thymine
and a pentose sugar. In case of RNA, thymine base
is replaced by uracil.
 CODON: series of three adjacent bases in one
polynucleotide chain of a DNA or RNA molecule
which codes for a specific amino acid.
 GENETIC CODE: the sequence of nucleotides in
a DNA or RNA molecule that determines the
amino acid sequence in the synthesis of proteins.

21.  A series of codons in part
of amRNA molecule. Each
codon consists of three
nucleotides, usually
representing a single
amino acid.
 The nucleotides are
abbreviated with the letters
A, U, G and C. This is
mRNA, which uses U (
uracil). DNA uses T (
thymine) instead.

22.  Congenital Disease.Congenital Disease.
Diseases which are present at birth.
 Hereditary/Familial Disease.Hereditary/Familial Disease.
Diseases which are derived from
one’s parents and transmitted in the
gametes through the generations.
Not all congenital diseases are genetic( congenital
Syphilis) and not all genetic diseases are congenital
(Huntington disease).

23. Mutations.

24. Definition:Definition:
 A permanent heritable change in a
gene or chromosomal structure and are
important in the causation of cancer
and some congenital diseases.

25. CAUSES OF MUTATIONSCAUSES OF MUTATIONS
 Chemicals
 Nitrous acid
 Alkylating agents
 5- bromouracil
 Antiviral drug iododeoxy uridine
 Benzpyrene in tobacco smoke

26.  X – rays & ultraviolet light
 Certain viruses such as bacterial virus

27. Types Of
Mutations.

28. 1.1. Point MutationPoint Mutation
2.2. DeletionDeletion
3.3. Frameshift MutationsFrameshift Mutations
4.4. Trinucleotide Repeat MutationsTrinucleotide Repeat Mutations

29. 1. Point Mutation:1. Point Mutation:
 Substitution of a single nucleotide base by a
different base.

31.  Sickle cell anaemia is the result
of a point mutation in codon
6 of the b-globin
gene resulting in the substitution
of amino acid glutamic acid by
valine .
 b-globin is a major component
of adult haemoglobin (HbA).
The single amino-acid
substitution results in a type of
haemoglobin termed HbS, which
has different properties from the
normal HbA. Under conditions of
low oxygen tension, or in an
atmosphere containing a low
oxygen level, the following

32.  (1)The haemoglobin agglutinates to form insoluble
rod-shaped polymers;
 (2)Red blood cells become distorted and sickle-
shaped ;
 (3)The sickle-shaped cells rupture easily causing
haemolytic anaemia;
 (4)The sickle shaped cells tend to block capillaries
interfering with the blood flow to various organs.

33. 2.2. Deletion Mutations:Deletion Mutations:
 Insertion or deletion of one or more base
pairs alters the reading frame of the DNA
strand.
 Deletion of one codon causing Cystic
Fibrosis.
 Deletion of 6 codons in the b-globin gene
resulting in a variant haemoglobin

36. 3.3. Frame shift mutations
 Frame shift mutations involve a deletion or insertion
of one or two base pairs within a coding sequence of a
gene.
 As the coding message is read in triplets codons and
deletions the reading frame of mRNA is altered
resulting in a non-sense sequence of amino acids.
 An example occurs in the b-globin gene in which one
nucleotide of codon 39 is deleted . The following
reading frame is completely altered and continues
until a stop codon is encountered.
 Duchenne muscular dystrophy are caused by frame
shift mutations in the dystrophin gene.

37. DeletionDeletion

38. InsertionInsertion
 Tay-Sachs disease is a rare inherited disorder that progressively
destroys nerve cells (neurons) in the brain and spinal cord

39. 3.3. Trinucleotide RepeatTrinucleotide Repeat
Mutations:Mutations:
 Trinucleotides are triplets of nucleotides that are
repeated in tandem many times over. The number of
repeats varies in different individuals.
 An example of trinucleotide repeats is - - - CAG
CAG CAG CAG CAG - - - -, and is designated as
(CAG)n where n is the number of repeats in the
particular individual.
 A form of A form of mutationmutation characterized by a stretch of three characterized by a stretch of three
nucleotidesnucleotides ( (codoncodon) repeated in multiple times in the) repeated in multiple times in the
DNA sequence.DNA sequence. e.g. Fragile X Syndrome(developmental
problems including learning disabilities and cognitive impairment. )

41. Classification Of GeneticClassification Of Genetic
Diseases:Diseases:
 Single Gene Defects/Mendelian Disorders.
 Disorders with Multifactorial or Polygenic
inheritance.
 Cytogenetic Disorders.
 Disorders showing atypical patterns of
inheritance.

42. Single gene disorders/Single gene disorders/
Mendelian DisordersMendelian Disorders
A genetic disease caused by a
single mutation in the structure
of DNA, which causes a single
basic defect with pathologic
consequences

43. According to the Patterns OfAccording to the Patterns Of
Inheritance, single gene disordersInheritance, single gene disorders
are:are:
 Autosomal Dominant Disorder.
 Autosomal Recessive Disorder.
 X-Linked Recessive Disorder.
 X-Linked Dominant Disorder.

44. AutosomalAutosomal
DominantDominant
Disorders.Disorders.

45. The pedigree on the right illustrates the
transmission of an autosomal
dominant trait.
Affected males and females have an
equal probability of passing on the
trait to offspring.
Affected individual's have one normal
copy of the gene and one mutant
copy of the gene, thus each offspring
has a 50% chance on inheriting the
mutant allele.
As shown in this pedigree,
approximately half of the children of
affected parents inherit the
condition and half do not.
 Individuals with these diseases
usually have one affected parent .*
 Autosomal Dominant
Conditions:
• Huntington Disease
• Acondroplasia (short-
limbed dwarfism)
• polycystic kidney
disease

46. Inheritance Pattern:Inheritance Pattern:
• Typical mating pattern isTypical mating pattern is
aa heterozygousheterozygous affectedaffected
individualindividual with awith a
homozygoushomozygous
unaffectedunaffected individual.individual.
• Both sexes areBoth sexes are
affected equally..affected equally..

47. Disorders:Disorders:

48. Marfan’s Syndrome:Marfan’s Syndrome:
genetic disorder that affects the body's
connective tissue.
Mutation in the fibrillin gene.
 Fibrillin important component of
microfibrils in Elastin.
Tissues affected are Skeleton, Eyes and
the CVS.
 C/F include tall stature, long fingers,
pigeon breast deformity, hyper-
extensible joints, high arched palate, BL
subluxation of lens, floppy Mitral valve,
Aortic aneurysm and dissection, defects
in skin, lungs.

49.  What is Fibrillin?
 Marfan syndrome is caused by changes in fibrillin
genes. Fibrillin is a glycoprotein, which is essential
for the formation of elastic fibers or microfibrils that
provide strength and flexibility to connective tissue.
 Connective tissue holds the body together and helps
control the growth and repair of tissues and organs.
 Fibrillin normally is abundant in the connective
tissue found in the aorta, in the ligaments that hold
the eye's lenses in place, in the bones and the lungs.

50.  How is Fibrillin related to Marfan syndrome?
 Marfan syndrome is caused by changes (mutations)
in one member of the pair of fibrillin genes. As a
result, the body produces fibrillin that does not
work and connective tissue that is not as strong as it
should be.
 The growth and development of the body are
affected, particularly in the connective tissues of the
aorta, eye and skin.
 It causes overgrowth of the long bones of the body,
resulting in tall height, long arms and legs, and a
weakened structural support in blood vessels, heart
valves, cartilage and ligaments.

51. Ehler-Danlos Syndrome(CutisEhler-Danlos Syndrome(Cutis
Hyperelastica):Hyperelastica):
 Characterized by defects in collagen
synthesis/connective tissue.
 Clinical Features include fragile, hyper-extensible
skin, hyper-mobile joints, rupture of internal
organs like the colon, cornea and large arteries,
poor wound healing.

53. AutosomalAutosomal
RecessiveRecessive
DisordersDisorders

54.  Recessive conditions are clinically
manifest only when an individual has
two copies of the mutant allele.
 When just one copy of the mutant
allele is present, an individual is a
carrier of the mutation, but does not
develop the condition.
 Females and males are affected
equally by traits transmitted by
autosomal recessive inheritance.
 When two carriers mate, each child
has a 25% chance of being
homozygous wild-type (unaffected); a
25% chance of being homozygous
mutant (affected); or a 50% chance of
being heterozygous (unaffected
carrier).
 Affected individuals are
indicated by solid black symbols
and unaffected carriers are
indicated by the half black
symbols.
 Autosomal recessive diseases:
• Cystic fibrosis
• Tay-Sachs
• hemochromatosis
• phenylketonuria (PKU)

55. Pattern Of Inheritance:Pattern Of Inheritance:
 Typical mating pattern is two
heterozygous unaffected
(carrier) individuals.
The trait doesnot usually affect
the parent, but siblings may
show the disease
Siblings have one chance in
four of being affected
 Both sexes affected equally.

56. Disorders:Disorders:

57. X-LinkedX-Linked
RecessiveRecessive
Disorders.Disorders.

58.  Most common X-linked
disorders.
 Usually expressed only in males.
 Rarely, due to random X-
inactivation, a female will express
disease, called manifesting
heterozygotes.
 An unaffected woman carries one
copy of a gene mutation for an X-
linked recessive disorder. She has
an affected son, an unaffected
daughter who carries one copy of
the mutation, and two unaffected
children who do not have the
mutation.

59. Pattern OfPattern Of
Inheritance:Inheritance:
• Disease usually passed on
from carrier mother.
• Expressed in male offspring,
females are carriers.
• Skipped generations are
commonly seen.
• In this case, Recurrence risk
is half of sons are affected,
half of the daughters are
carriers.

60. Recurrence risk:
All the daughters are
terozygous carriers and all
e sons are homozygous
rmal.
here is no father to son
nsmission, but there is
her to daughter and
other to daughter and son
nsmission.
f a man is affected with an
linked recessive
ndition, all his daughter
ll inherit one copy of the
utant allele from him.

61. Disorders:Disorders:

62. X-linked DominantX-linked Dominant
DisorderDisorder

63.  Because the gene is located on
the X chromosome, there is no
transmission from father to son,
but there can be transmission
from father to daughter (all
daughters of an affected male
will be affected since the father
has only one X chromosome to
transmit).
 Children of an affected woman
have a 50% chance of inheriting
the X chromosome with the
mutant allele.
 X-linked dominant disorders are
clinically manifest when only
one copy of the mutant allele is
present.
X-linked Dominant Disorders
• some forms of retinitis
pigmentosa
• Chondrodysplasia
Punctata
• hypophosphatemic
rickets

64. DISORDERS WITHDISORDERS WITH
MULTIFACTORIALMULTIFACTORIAL
(POLYGENIC)INHERITANCE(POLYGENIC)INHERITANCE
 Multifactorial inheritance, also called complex
or polygenic inheritance.

65.  Multifactorial inheritance disorders are caused
by a combination of environmental factors and
mutations in multiple genes(genetic factor).
For example, different genes that influence
breast cancer
 susceptibility have been found on
chromosomes 6, 11, 13, 14, 15, 17, and 22.
Some common chronic diseases are
multifactorial disorders.

66.  Rate of recurrence is 2 to 7%Rate of recurrence is 2 to 7%

67. COMMON DISEASES ASSOCIATED with DISORDERS WITHCOMMON DISEASES ASSOCIATED with DISORDERS WITH
MULTIFACTORIAL INHERITANCEMULTIFACTORIAL INHERITANCE
 Diabetes mellitusDiabetes mellitus
 HypertensionHypertension
 GoutGout
 Cleft lip and palateCleft lip and palate
 SchizophreniaSchizophrenia
 Bipolar disorderBipolar disorder
 Congenital heart diseaseCongenital heart disease
 Skeletal abnormalitiesSkeletal abnormalities
 Neural tube defectsNeural tube defects
 Coronary artery diseaseCoronary artery disease

68. Cytogenetic
Disorders.

69.  Cytogenetic disorders may result
from structural or numeric
abnormalities of chromosomes
 It may affect autosomes or sex
chromosomes

70. Numeric Abnormalities:
 Normal Chromosomal number is 46. (2n=46). This
is called euploid state. (Exact multiple of haploid
number).
 Polyploidy: posession of more than two sets of
homologous chromosomes. Chromosomal numbers
like 3n or 4n. (Incompatible with life); generally
results in spontaneous abortion
 Aneuploidy: Any Chromosomal number that is not
an exact multiple of haploid number . E.g 47 or 45.

71. Aneuploidy:
 Most common cause is
nondisjunction (attach)of
either a pair of homologous
chromosomes during
meiosis I or failure of sister
chromatids to separate
during meiosis II.
 The resultant gamete will
have either one less
chromosome or one extra
chromosome.

72.  Fertilization of such gamete will
result in zygote being either trisomic
( 2n+1 ) or monosomic ( 2n-1 ).
 Monosomy in autosomes is
incompatible with life.
 Trisomy of certain autosomes and
monosomy of sex chromosomes is
compatible with life.

73. Mosaicism:
 A person or a tissue that contains two or more
types of genetically different cells.
 Mosaicism is caused by an error in cell division
very early in the development of the unborn baby.
 Examples of mosaicism include:
 Mosaic Down Syndrome
 Mosaic Klinefelter Syndrome
 Mosaic Turner Syndrome

74. Structural Abnormalities:
 Usually result from chromosomal
breakage, resulting in loss or
rearrangement of genetic material.
 Patterns of breakage:
• Translocation.
• Isochromosomes.
• Deletion.
• Inversions.
• Ring Chromosomes.

75. TRANSLOCATIONTRANSLOCATION
 when a part of one chromosome is transferred
to another chromosome
 Translocations are indicated by t
 E.g. 46,XX,t(2;5)(q31;p14)
 There are two type of translocation.
1. In a reciprocal translocation,
segment from two different
chromosomes have been
exchanged

76. 2. Centric fusion type or robertsonian
translocation:
 The breaks occur close to the centromere, affecting
the short arms of both choromosomes
 Transfer of the chromosome leads to one very large
and one extremely small chromosome
 The short fragments are lost, and the carrier has 45
chromosomes
 These occur with chromosomes 13, 14, 15, 21, and
22
 Such loss is compatible with survival

78. ISOCHROMOSOMESISOCHROMOSOMES
 Result when one arm of a chromosome is lost and the
remaining arm is duplicated, resulting in a chromosome
consisting of two short arms only or of two long arms.

79. DELETIONDELETION
 Loss of a portion of chromosome
 This can be terminal (close to the end of the
chromosome on the long arm or the short
arm), or it can be interstitial (within the long
arm or the short arm).
 A ring chromosome is a variant of deletion.It
occurs when break occurs at both the ends
of chromosome with fusion of the damaged
ends.

80. INVERSIONS
 A portion of the chromosome has broken off ,
turned upside down and reattached, therefore
genetic material is inverted.
 Occur when there are two breaks within a single
chromosome with inverted reincorporation of the
segment.
 Since there is no loss or gain of chromosomal
material, inversion carriers are normal.
 An inversion is paracentric if the inverted segment
is on the long arm or the short arm .
 The inversion is pericentric if breaks occur on both
the short arm and the long arm .

83.  Ring Chromosome
 A portion of a chromosome has broken off
and formed a circle or ring . This can
happen with or without loss of genetic
material.

84. General Features of Cytogenetic Disorders:
 Associated with absence, excess, or
abnormal rearrangements of
chromosomes.
 Loss of genetic material produces more
severe defects than does gain.
 Abormalities of sex chromosomes
generally tolerated better than those of
autosomes.

85.  Sex chromosomal abnormalities are
usually subtle and are not detected at
birth.
 Most cases are due to de novo changes
(i.e. parents are normal and recurrence
in siblings is low).
 Defn
of de novo change: An alteration in a gene that is present
for the first time in one family member as a result of a mutation
in a germ cell (egg or sperm) of one of the parents or in the

86. Cytogenetic
Disorders involving
Autosomes.

87. Trisomy 21/Down’s Syndrome:
 Most common chromosomal disorder.
 Down syndrome is a chromosomal
abnormality characterized by the
presence of an extra copy of genetic
material on the 21st
chromosome
 Trisomy 21 is caused by a meiotic
nondisjunction (attach) event.

88.  With nondisjunction, a gamete (i.e., a sperm
or egg cell) is produced with an extra copy of
chromosome 21; the gamete thus has 24
chromosomes
 When combined with a normal gamete from
the other parent, the embyo now has 47
chromosomes, with three copies of
chromosome 21.
 About 4% of cases are due to Robertsonian
translocations.
 Maternal age has a strong influence

89.
Karyotype for trisomy Down syndrome. Notice the three copies of
chromosome 21

90. Other Trisomy Syndromes:
 Trisomy 18 : Edwards Syndrome.
 Trisomy 13 : Patau Syndrome.

91. Cytogenetic Disorders
involving Sex
Chromosomes

92.  Extreme karyotypic variations seen
frequently with Sex Chromosomes, with
females having 4-5 extra X Chromosomes.
 Males with 2 to 3 Y chromosomes have also
been identified.

93. Klinefelter’s Syndrome:
 Defined as Male Hypogonadism (doesnot
produce enough testosterone), develops when
there are at least two X chromosomes
and one or more Y chromosomes.
 Usual karyotype is 47,XXY. The extra X
may be maternal or paternal.
 A A karyotypekaryotype (Greek karyon = kernel, seed or nucleus) is (Greek karyon = kernel, seed or nucleus) is
the number and appearance of chromosomes in thethe number and appearance of chromosomes in the
nucleus of a eukaryotic cell. nucleus of a eukaryotic cell.

94.  Results from nondisjunction of sex
chromosome during meiosis.
 Risk factors include advanced maternal
age and a history of exposure to
radiation in either parent.

95. Clinical Manifestations:
 Increase in body length between soles and
pubis.
 Reduced facial, body and pubic hair.
Gynecomastia.
 Testicular atrophy.
 Infertility.
 Mild mental retardation.

96. Turner Syndrome:
 Primary hypogonadism in females
 Results from partial or complete
monosomy of the X chromosome.

97.  Most common cause is absence of one
X chromosome.
 Less commonly, mosaicism(addition no. of
chromosome), or deletions on the short
arm of the X chromosome.

99. THANK YOUTHANK YOU