Genetics/prosthodontic courses

 Genetics ,studies how living organisms inherit
many of the features of their ancestors –
for example, children usually look and act like
other people in their family.
 Genetics tries to identify which features are
inherited, and work out the details of how
these features are passed from generation to
generation.
 It is a rapidly developing science that has
reached an advanced level of genetic
selection and cloning.
www.indiandentalacademy.com

 The branch of biology that deals with the
facts & laws of heredity & inherited variations.
The science concerned with the structure &
function of all genes in different organisms

 William Bateson
was a British
geneticist. He was
the first person to
use the term
“genetics” to
describe the study
of heredity and
biological
inheritance.

 Bateson was the first to suggest the word
"genetics" (from the Greek genno, i.e. to give
birth) to describe the study of inheritance and
the science of variation .
 Bateson first used the term "genetics" publicly
at the Third International Conference
on “Plant Hybridization” in London in
1906.

 In 1909,Danish botanist Wilhelm Johannsen
proposes the term "gene" (from the Greek
word "genos" which means "birth")
 1882, German biologist
Walter Fleming, by staining
cells with dyes, discovers rod-
shaped bodies he calls
"chromosomes."

 Oswald Avery, Colin
MacLeod, and
Maclyn McCarty
report evidence
that, at least in
bacteria, the
molecule that
carries genetic
information is
deoxyribonucleic
acid (DNA)

 1953,Francis Crick and
James Watson determine
that the structure of the
DNA molecule is a
double helix composed
of strings of nucleotides
and that two parallel
strands formed by sugar
and phosphate molecules
are joined together by
the bonding of specific
pairs of nitrogenous
bases. They share a
Nobel Prize for this in
1962.

 1955
 Joe Hin Tijo
determines that the
number of
chromosomes in
humans is 46.
(For 30 years, the
number was
believed to be 48.)

 1961,Sydney Brenner,
Francois Jacob, and
Matthew Meselson
identify the role of
Ribonucleic Acid (RNA).
They determine that
messenger RNA
(mRNA) is the molecule
that carries the genetic
information from DNA
in the nucleus out into
the cytoplasm and that
the cell ultimately uses
mRNA to make specific
proteins.

 Gregor Johann
Mendel, an Austrian
monk & scientist,
publishes his
findings on the laws
of inheritance based
on experiments,
begun in 1857, with
pea plants

 Published in his paper “Experiment on plant
hybridization” in 1865 to the Brunn Natural
History society.
 He laid the foundation for studies of
inheritance in the twentieth century and
beyond.
 “Father of genetics."

 Allele: One alternative of a pair or group of
genes that could occupy a specific position
on a chromosome.
 Chromosome: A linear strand of DNA
harboring many genes.
 DNA: Deoxyribonucleic acid; the molecule in
which genetic information is encoded.
 Gene: A unit of genetic information that
occupies a specific position on a
chromosome and comes in multiple versions
called alleles.

 Dominant: An allele producing the same
phenotypic effect whether inherited
heterozygously or homozygously; an allele
that "masks" a recessive allele.
 Recessive: An allele producing no phenotypic
effect when inherited heterozygously and
only affecting the phenotype when inherited
homozygously; an allele "masked" by a
dominant allele.

 Genotype: The genetic constitution of an
organism.
 Phenotype: The physical or observable
characteristics of an organism.
 Heterozygous: Having a genotype with two
different and distinct alleles for the same
trait.
 Homozygous: Having a genotype with two of
the same alleles for a trait.

 Mendel carried out his experiments, on the
common garden pea, Pisum sativum.
 His work was not something that was never
done, but he was the first to notice that the
inheritance units obeyed certain statistical
laws.

 His studies were based on seven traits of
peas. The seven traits were qualitative (they
could be measured and a value assigned);
therefore, specified qualities could be
assigned to each plant.
 These characteristics were visible and it was
through them that he could study the effects
of reproduction.

 For each of the seven pair of characters,
plants with an alternative trait were used as
female & other as male.
 The plants of unlike characters with which
hybridization is first made constitute parent
generation ( P1 Generation)

 Two contrasting varieties were crossed & the
first set of offsprings was known as the first
filial or F1 Generation
 The progeny of F1 plants obtained due to
self-fertilization represents the second filial
generation or F2 Generation

 Plants obtained from the crossing of two
individuals are known as HYBRIDS & the
process known as HYBRIDIZATION
 In F1 generation, the offspring always
resembled only one parent. The character
which was manifested was referred to as
Dominant & other as Recessive

 Based on observations of his experiments on
garden pea.
 He drew some important conclusions & given
three laws:

 Mendel’s first experiment with varieties of
garden pea that differed only in one visible
character : monohybrid experiment
 For example, when testing the shape of the
seed, crossing one pure-breed round seed
with a pure-breed wrinkled seed, all of the
offspring were round.

 The one trait - in this case is a wrinkled seed
- not expressed in the offspring he called it
recessive trait. In each case of this crosses,
the round trait was dominant over the
wrinkled trait and is said to be the dominant
trait. This conclusion is now referred to as
Mendel's Law of Dominance

 Law of Segregation:
when a pair of allelomorphs are brought
together in the hybrid F1, they remain
together in the hybrid without blending but
separate completely & purely during gamete
formation.

 Mendel experimented on parents differed in
two pairs of characters. Such a cross is known
as DIHYBRID CROSS.
 Also known as "Inheritance Law", states that
the inheritance pattern of one trait will not
affect the inheritance pattern of another.

 While Mendel's experiments with mixing one
trait always resulted in a 3:1 ratio between
dominant and recessive phenotypes, his
experiments with mixing two traits (dihybrid
cross) showed 9:3:3:1 ratios But the 9:3:3:1
shows that each of the two genes are
independently inherited with a 3:1 ratio.

 CONCLUSION:
That different traits are inherited
independently of each other, so that there is
no relation.

 Dominance is absent i.e. the f1 exhibit
neither of two phenotypes presented in
parents. The hybrid individual resemble
neither parent & are intermediate between
those of two parents
 Thus both alleles have almost equal effect on
the phenotype, resulting in intermediate
character of hybrid

 Both allelic genes of a genetic genes of a
genetic triad, one equally expressive i.e. the
dominant character is not able to suppress
the recessive character & thus both the
characters appears side by side F1 hybrids

 Human chromosome preparations first made
by FLEMMING (1882)
 In 1903, Sutton and Boveri , independently,
proposed that it was the interaction between
these chromosomes that lead to the
phenomenon of inheritance.
 First determination of chromosome no.:
WINIWARTER (1912), reported 2n=48.
 Human Y chromosome: PAINTER (1923)

 Correct human chromosome 2n=46: TIJO &
LEVAN (1956)
 Autosome term: MONTGOMERY (1904)
 Sex chromosome / heterosomes: WILSON
(1906)
 Chromosomes are organized structures of
DNA and proteins that are found in cells
 Cells contained a nucleus and the nucleus
had threadlike substances called
chromosomes

 The word chromosome comes from (chroma,
color) and (soma, body) due to their property
of being stained very strongly by some dyes.
 The total complement of genes in an
organism or cell is known as its genome,
which may be stored on one or more
chromosomes; the region of the chromosome
at which a particular gene is located is called
its locus.

 A chromosome is formed
from a single DNA
molecule that contains
many genes. A
chromosomal DNA
molecule contains three
specific nucleotide
sequences which are
required for replication: a
DNA replication origin; a
centromere to attach the
DNA to the mitotic
spindle.; a telomere
located at each end of
the linear chromosome.

 There are 46 chromosomes in the normal
human – 23 pairs.
 The members of each pair match with respect
to the genetic information they carry.
 One chromosome of the pair is inherited from
the father, and one from the mother, and
further, one is transmitted to the child.
 22 pairs are alike in males and females –
known as autosomes
 1 pair differs – the sex chromosomes.

 the female sex chromosomes – there are two
X chromosome.
 In the male, there is one X chromosome and
one Y chromosome which is smaller than the
X chromosome.
 There are 2 types of cell division –
 Mitosis – normal cell division, by virtue of
which the body grows – it results in 2
daughter cells, identical to the parent cell in
genetic makeup, and number of
chromosomes.

 Meiosis – This results in the production of
reproductive cells (gametes). Each of which
have only 23 chromosomes.

 Chromosomal abnormalities can be of the
following types:-
 Abnormality in – Autosomes
- Sex chromosomes
 Abnormality in – Number
- Structure

 Loss or gain of a single chromosome is
known as aneuploidy
 Polyploidy – is a gain of the whole
chromosome set – ie – 3N or 4N number of
chromosomes.
 Monosomy – is the loss of a single
chromosome.

 Trisomy – This is the gain of a single
chromosome.
 Lejeune (1959) was the first to show that
patients with Down’s syndrome had an extra
Chromosome 21.
 The main cause of trisomy is the failure of
homologous chromosomes to separate
during meiosis. This is known as non-
disjunction.

 Deletions: A portion
of the chromosome
is missing or
deleted. Known
disorders include
Wolf-Hirschhorn
syndrome, which is
caused by partial
deletion of the
short arm of
chromosome 4.

 Duplications: A portion
of the chromosome is
duplicated, resulting in
extra genetic material.
Known disorders
include Charcot-Marie-
Tooth disease type 1A
which may be caused
by duplication of the
gene encoding
peripheral myelin
protein 22 (PMP22) on
chromosome 17.

 Inversions: A
portion of the
chromosome has
broken off, turned
upside down and
reattached,
therefore the
genetic material is
inverted.

 Translocations:
When a portion of
one chromosome is
transferred to
another
chromosome. There
are two main types
of translocations.

 In a reciprocal
translocation,
segments from two
different
chromosomes have
been exchanged.

 Robertsonian
translocation, an
entire chromosome
has attached to
another at the
centromere; these
only occur with
chromosomes 13,
14, 15, 21 and 22.

 If a deletion occurs
at 2 ends of a
chromosome, such
that the resultant
ends have
complementary
base pairs, they
tend to join, and
form a ring
chromosome.

 Abnormality of number
 Seen in many syndromes –
Klinefelter’s syndrome – XXY
Turner’s syndrome – females with only 1 X
chromosome
Multiple X – females with 3 or 4 X
chromosomes
XYY males

 Abnormalites of Structure
Isochromosome X – a long X chromosome –
which results from deletion of the short arms
of the X chromosome and duplication of the
long arm.

 A gene can be defined as a region of DNA
that controls a hereditary characteristic. It
usually corresponds to a sequence used in
the production of a specific protein or RNA.
OR
 Genes are instruction manuals for our bodies.
They are the direction for building all proteins
that make our bodies function.

 A gene carries biological information in a
form that must be copied and transmitted
from each cell to all its progeny. This includes
the entire functional unit: coding DNA
sequences, non-coding regulatory DNA
sequences, and introns.
 1910,Thomas Hunt Morgan argued that
genes are on chromosomes.

 genes are specific
segments of DNA
located along
chromosomes.
Because different
genes are made up of
different sequences
of bases, different
genes contain
different information
about the body’s
traits and functions.

 Genes have both
coding and non-
coding sequences.
Coding sequences
are those sequences
that are used as a
template for protein
synthesis during the
process of
translation. The
coding sequences of
a gene are called
exons.

 Non-coding sequences
do not provide
information about the
protein for which the
gene codes, but these
sequences often play
important roles in
regulating gene
expression and protein
synthesis. There are
different types of non-
coding sequences
within each gene,
including introns,
promoters

 the coding sequences, called exons, are
separated by intervening, non-coding
sequences called introns.
 Whereas exons contain genetic information
that will be used to make the protein, introns
do not.
 Both exonic and intronic sequences are
transcribed into an RNA molecule, but only
exons contribute to the protein.

 Before being translated into a protein, the
RNA undergoes splicing, a process whereby
the intronic RNA sequences are removed and
discarded and the exonic RNA sequences are
joined together to give a shorter, “mature”
messenger RNA (mRNA) molecule.
 Splicing occurs specifically at the exon/intron
boundaries; these boundaries are called the
splice junctions, and they are characterized
by particular sequences.

 Like introns, the promoter of a gene also
does not code for protein. However, the
promoter is very important for regulating
whether a gene is expressed or not
expressed. When a gene is expressed (turned
“on”), the gene is transcribed into an mRNA
molecule.
 When a gene is not expressed (turned “off”),
the gene is not transcribed; an mRNA
molecule is not made.

 The process of producing a biologically
functional molecule of either RNA or protein
is called gene expression, and the resulting
molecule itself is called a gene product.

 Defined as a sudden inheritable genetic
change, altering the genetic message of a
cell.
 Heritable change in the structure of a gene/
chromosome/ change in chromosome no.
 Mainly responsible for variations in organisms
 First seen in 18th century : SETH H WRIGHT
(new England farmer) in a male ANCON SHEEP
 Term first used: Dutch Botanist HUGO DE
VRIES (1901)

 Depending on kind of cell in which mutation
occurs
 Somatic: malignant or cancerous growth is a
kind of mutation which is localized & not
transmitted to offsprings
 Germinal: occurs in germ cells during
gametogenesis

 AMATTO (1950)
 Gene mutation
 Chromosomal mutation
 Genomatic mutation (change in no.)

 Change in gene duplication: gene mutation
 Mutant & original gene located at same fixed
point on a particular chromosome & since
gene is situated at a fixed point; POINT
MUTATION
 Mostly include alteration in the sequences of
nucleotides in the nucleic acid which form the
genetic material

 Change in chromosome structure/
morphology: chromosomal mutation
 Do not involve changes in the no. of
chromosomes but result from changes in the
no. or sequence of genes on chromosomes

 Changes in chromosome no.: genomatic
mutation(heteroploidy)
 2 types
 Aneuploidy
 euploidy

 It is the presence of a chromosome no. which
is different then the multiple of basic
chromosome no.
 Either loss (hypoploidy) or addition
(hyperploidy) of one or more chromosome
occurs.

 Mutations which involve addition of complete
set of chromosome.

 Deoxyribonucleic acid (DNA) is the
macromolecule that stores the information
necessary to build structural and functional
cellular components.
 DNA normally exists as a double-stranded
molecule, coiled into the shape of a double-
helix.

 Double helix
model has two
strands of DNA with
nucleotides
pointing inwards,
each matching a
complementary
nucleotide on other
strand .

 This shows that
genetic information
exists in the
sequence of
nucleotides on each
strand of DNA.
 Information is held
in the sequence of
repeating units
along the DNA
chain.

 These units are four
types of nucleotides
 A,G,T,C
 They pair up to
hold the two
strands together
 An A nucleotide
goes opposite to T
& G opposite to C.
 This exact pairing:
Base pairing

 The 2 chains are held together by hydrogen
bonds between the nitrogenous bases which
point in towards the centre of the helix.
 Two chains have opposite orientation, and
are said to be in an anti – parallel orientation

 When a gene is read by a cell the DNA
sequence is copied into a very similar
molecule called RNA: transcription
 This messenger RNA molecule is then used
to produce a corresponding amino acid
sequence through a process called translation

 When genes are passed from a parents to a
child they are copied. Parents keep the same
no. of genes as they had before & just passes
on the new copies to their offsprings.
 Genes are always copied each time a cell
divides into two new cells.
 Process which copies DNA: replication

 Inheritance is the process by which
characteristics of parents are transferred to
offspring.
 Children inherit traits, disorders, and
characteristics from their parents. Children
tend to resemble their parents especially in
physical appearance. However they may also
have the same mannerisms, personality, and
a lot of the time the same mental abilities or
disabilities.

 Genetic disorders can be of 3 main types :-
 Single gene disorders – these occur due to
mutations of single genes. They show typical
pedigree patterns and are rare - 1 in 2000 or
less.
 Chromosome disorders – the disorder occurs
due to an excess or deficiency of whole
chromosomes or chromosome segments.
They show characteristic features, and are
relatively more common than single gene
disorders - 7 in 1000 births.

 Multifactorial disorder – These are caused
due to a combination of genetic and
environmental factors. They are the most
common of the genetic disorders and do no
show the typical pedigree patterns on single
gene disorders.

 Single Gene disorders.
 These can be –
 Autosomal dominant
 Autosomal recessive
 Sex linked dominant
 Sex linked recessive

 Two copies of the gene must be mutated for
a person to be affected by an autosomal
recessive disorder. An affected person usually
has unaffected parents who each carry a
single copy of the mutated gene (and are
referred to as carriers). Two unaffected
people who each carry one copy of the
mutated gene have a 25% chance with each
pregnancy of having a child affected by the
disorder.
 Eg. Sickel cell anaemia, Muscular dystrophy

 Only one mutated copy of the gene is needed
for a person to be affected by an autosomal
dominant disorder. Each affected person
usually has one affected parent. There is a
50% chance that a child will inherit the
mutated gene.
 Eg. Achondroplasia
Osteogenesis imperfecta
some forms of Amelogenesis imperfecta

 X-linked dominant disorders are caused by
mutations in genes on the X chromosome.
 Females are more frequently affected than
males, and the chance of passing on an X-
linked dominant disorder differs between
men and women.
 The sons of a man with an X-linked dominant
disorder will not be affected, and his
daughters will all inherit the condition.

 A woman with an X-linked dominant disorder
has a 50% chance of having an affected
daughter or son with each pregnancy

 X-linked recessive disorders are also caused
by mutations in genes on the X chromosome.
Males are more frequently affected than
females, and the chance of passing on the
disorder differs between men and women.
 The sons of a man with an X-linked recessive
disorder will not be affected, and his
daughters will carry one copy of the mutated
gene

 With each pregnancy, a woman who carries
an X-linked recessive disorder has a 50%
chance of having sons who are affected and a
50% chance of having daughters who carry
one copy of the mutated gene.

 Y-linked disorders are caused by mutations
on the Y chromosome.
 Only males can get them, and all of the sons
of an affected father are affected.
 Females are not affected as they do not have
a Y chromosome.

 This type of inheritance, also known as
maternal inheritance, applies to genes in
mitochondrial DNA.

 Intermediate inheritance.
Some mutant genes are only partially
expressed in the heterozygote. This is known
as incomplete dominance, or intermediate
inheritance.
 An example is sickle cell anaemia. A person
homozygous for the mutant gene, shows
typical sickling of the RBCs. The heterozygote
on the other hand shown normal RBCs in
normal condition.

 But if the heterozygote is exposed to low
oxygen tension, as in high altitude travel etc,
the RBCs change from the normal shape to
sickle shape. These people are said to have
the sickle cell trait.

 Codominance.
In some cases, the alleles for a particular
characteristic may be different, but both may
be expressed. This is known as co-
dominance. E.g. – both the antigens A and B
are present in blood group AB.

 A person's cells hold the exact genes that
originated from the sperm and egg of his
parents at the time of conception. The genes
of a cell are formed into long strands of DNA.
Most of the genes that control characteristic
are in pairs, one gene from mom and one
gene from dad

 Everybody has 22 pairs of chromosomes
(autosomes) and two more genes called sex-
linked chromosomes. Females have two X
(XX) chromosomes and males have an X and a
Y (XY) chromosome.

 However, the modern science of genetics,
which seeks to understand the process of
inheritance, only began with the work of
Gregor Mendel in the mid-nineteenth
century.
 Although he did not know the physical basis
for heredity, Mendel observed that organisms
inherit traits in a discrete manner—these
basic units of inheritance are now called
GENES

 Discovered in 1983,by WALTER JACOB GEHRING
homeobox genes, and the proteins they encode,
the homeodomain proteins, have turned out to
play important roles in the developmental
processes of many multicellular organisms.
 A homeobox is a DNA sequence found within
genes that are involved in the regulation of
development (morphogene) of animals,
fungi and plants. Genes that have a homeobox
are called homeobox genes and form the
homeobox gene family.

 A homeobox is a DNA sequence found within
genes that are involved in the regulation of
development (morphogenesis) of animals,
fungi and plants. Genes that have a
homeobox are called homeobox genes and
form the homeobox gene family.
 Homeobox genes encode transcription
factors which typically switch on cascades of
other genes. The homeodomain binds DNA in
a specific manner

 Hox genes: Hox genes are a subgroup of
homeobox genes. In vertebrates these genes
are found in gene clusters on the
chromosomes. In mammals four such clusters
exist, called Hox clusters. The gene name
"Hox" has been restricted to name Hox
cluster genes in vertebrates. Only genes in
the HOX cluster should be named Hox genes.
So note: homeobox genes are NOT Hox
genes, Hox genes are a subset of homeobox
genes.

 homeodomain: a DNA-binding domain,
usually about 60 amino acids in length,
encoded by the homeobox.
 homeobox: a fragment of DNA of about 180
basepairs (not counting introns), found in
homeobox genes.

 These genes, also known as HOX genes, form
hox codes, which specify the position a cell
should occupy, and the structure that cell
should develop into. Interestingly, they have
an anterior-posterior arrangement. The
anterior genes coding for anterior structures,
and posterior genes coding for posterior
structures.

 Humans generally contain homeobox genes
in four clusters:
HOXA (or sometimes HOX1) - chromosome 7
HOXA1, HOXA2, HOXA3, HOXA4
 HOXB - chromosome 17 HOXB1, HOXB2,
HOXB3, HOXB4
 HOXC - chromosome 12 HOXC4, HOXC5,
HOXC6
 HOXD - chromosome 2 HOXD1, HOXD3,
HOXD4, HOXD8

 Mutation
 Mutations to homeobox genes can produce
easily visible phenotypic changes.
 Duplication of homeobox genes can produce
new body segments

 Amelin & enamelin: enamel protein
 BMP 2&4: tooth formation
 DSPP: defective mineralization of teeth
 BSP: mineralization of bone & cementum
 MSX 1&2: incisors
 BARX & DLX-2: molars

 Dr. V.Singh, Dr. D.K.Jain, Book Of Biology, 3rd
edi., 2001, Pgs. 781, 782,819-825
 Richard Tencate, Oral Histology-
Development, Structure & Function,5th edi.,
Mosby
 Inderbir Singh, Human Embryology,7th edit.,
MacMillan
 http://faculty.uca.edu.com
 www. Wikipedia.co

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