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INTRODUCTION AND SCOPE OF GENETICS
The branch of science which deals with the mechanism of heredity and causes of variations in
living beings (viruses, bacteria, plants and animals) is known as genetics
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Although the science of genetics is relatively new, people have understood the hereditary nature
of traits and have “practiced” genetics for thousands of years
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Today, the
major crops and animals used in agriculture have undergone extensive genetic alterations to
greatly increase their yields and provide many desirable traits, such as disease and pest
resistance, special nutritional qualities, and characteristics that facilitate harvest
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Today, genetically engineered corn,
soybeans, and other crops constitute a significant proportion of all the food produced
worldwide
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Numerous drugs and food additives are synthesized by fungi and bacteria that have been
genetically manipulated to make them efficient producers of these substances
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Growth hormone, insulin, and clotting factor
are now produced commercially by genetically engineered bacteria
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It also plays a critical role in medicine
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● Advances in molecular genetics have allowed important insights into the nature of cancer
and permitted the development of many diagnostic tests
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BRANCHES OF GENETICS
Genetic analysis is practiced at different levels
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Another type of genetic analysis follows in the footsteps of Watson and Crick
and the army of people who have worked on the various genome projects by focusing on the
molecular makeup of the genetic material
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All these levels of genetic analysis
are routinely used in research today
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It is further divided into cytogenetic,
developmental genetics, forward genetics and reverse genetics so on
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During this time, geneticists pursued their science by analyzing the outcomes
of crosses between different strains of organisms, much as Mendel had done in his work with
peas
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Because these studies emphasize the transmission of genes and chromosomes from one
generation to the next, they are often referred to as transmission genetics
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It also studies the
nature of the genetic material—how it controls traits and how it mutates
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Since
recombination is one of the sources for releasing hereditary variation, its study at all
levels particularly for preparation of linkage maps (also including molecular mechanism

of recombination and preparation of molecular maps) has been undertaken in some
detail;
(iii)

Non-Mendelian genetics: It involves a study of the role of cytoplasm and its organelles
(particularly chloroplasts and mitochondria) in heredity
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In a broad sense, these
may Include both chromosomal changes (structural and numerical) and also gene
mutations

MOLECULAR GENETICS
The goal of molecular genetics, is to understand the molecular features of DNA and the features
underlie the expression of genes
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In many cases, researchers analyze the effects of gene mutations that
eliminate the function of a gene
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In view of
this, structure and function of gene and the regulation of its activity have been studied in
considerable detail
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The replication, expression, and mutation of genes could now be studied at the molecular level
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Knowledge of a DNA sequence and comparisons to other DNA
sequences allow a geneticist to define a gene chemically
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But the molecular approach to genetic
analysis is much more than the study of DNA sequences
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Whole genes, or pieces of genes, can be excised from one DNA molecule and
inserted into another DNA molecule
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Minute
quantities of a particular gene can be generated in the laboratory in an afternoon
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This artful
manipulation has allowed researchers to study genetic phenomena in great detail
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In addition, advances within molecular genetics have shed considerable light on the areas of
transmission and population genetics
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Furthermore, discoveries within molecular genetics have had
widespread applications in agriculture, medicine, and biotechnology
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Geneticists seek to
document the variability and to understand its significance
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If they do, the population is evolving
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It is also
useful in the effort to understand the inheritance of complex traits, such as body size or disease
susceptibility
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The foundations of population genetics arose during the first few decades of the twentieth
century
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Mendel’s work and that of many succeeding geneticists gave insight into the nature of genes and
how they are transmitted from parents to offspring
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To
relate these two phenomena, population geneticists have developed mathematical theories to
explain the prevalence of certain alleles within populations of individuals
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Population geneticists are
particularly interested in genetic variation and how that variation is related to an organism’s
environment
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The following are some general concepts in population genetics:
● Why two or more different alleles of a gene maintained in a population
● Factors that alter the prevalence of alleles within a population
● Contributions of genetics and environment in the outcome of a trait
● Genetics and the environmental influence in quantitative traits, such as size and weight
● Factors that influence the process of evolution
● Evolution at molecular level

DEVELOPMENTAL GENETICS

The field of developmental genetics, or the study of how genes influence the developmental
processes of an organism, has been influenced by new technologies and by interactions with
other fields of study throughout its history
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It describes the reorganization of the embryo via different types of morphogenetic and cellular
movements that result in the foundation of organ systems, and discusses the many signaling and
adhesion molecules that are involved in regulating these complex processes
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It provide a better understanding of the critical embryonic and prenatal developmental processes
that are undamental to the normal development of animals, including humans
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Each multicellular organism begins as a single cell that has the potential to develop into any cell
type
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The results of early
cloning experiments demonstrated that this process arises from differential gene expression
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• In the early Drosophila embryo, determination is effected through a cascade of gene control
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These genes are expressed in the female parent and produce RNA and
proteins that are deposited in the egg cytoplasm
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The dorsal–ventral axis is
defined by a concentration gradient of the Dorsal protein, and the anterior–posterior axis is
defined by concentration gradients of Bicoid and Nanos proteins
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The gap genes establish large sections of the embryo, the pair-rule genes affect
alternate segments, and the segment-polarity genes affect the organization of individual
segments
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All these genes
contain a consensus sequence called a homeobox that encodes a DNA-binding domain; the
products of homeotic genes are DNA-binding proteins that regulate the expression of other
genes
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• Apoptosis, or programmed cell death, plays an important role in the development of many
animals
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Apoptosis is a highly regulated process that depends on
caspases—proteins that cleave proteins
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In plants also developmental mutants for flower development (e
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stamens modified into petals)
have been isolated and studied leading to the isolation of genes controlling development of floral
organs

in plants

like Arabidopsis and Antirrhinum
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For regulating the developmental process, temporal genes have been identified in
several cases
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FORWARD GENETICS AND REVERSE GENETICS
(Reverse genetics (from DNA to phenotype) as opposed to classical forward genetics (from phenotype to DNA)
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This has been suggested in contrast to
forward genetics, where genes are mapped on the basis of phenotype, using the techniques of
classical genetics
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It has been argued that in forward genetics, we start the study on the basis of
phenotype, leading ultimately to the study of DNA sequences comprising the gene for this
phenotype
...
Therefore, Paul Berg (Nobel Laureate) in a recent report
(1991) suggested that the usage of the term reverse genetics be restricted to those studies, where
we start the study with a DNA segment with unknown phenotypic effect, introduce this DNA
(without any alteration or after modification) into a plant or an animal and then study its

phenotypic effect
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These
techniques of reverse genetics will be increasingly used in future leading to significant advances
in our knowledge of genetics
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Heredity is the cause of similarities between individuals
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Variation is the cause of differences between individuals
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Variations on the
other hand, are mainly of two types, namely (i) hereditary and (ii) environmental
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Such variations are found not only
in progenies of different parents but also among progeny from the same parents
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Identical twins, however, is an exception, where
no hereditary variation exists
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Stated another way, each genotype may produce several different phenotypes,
depending on the environmental conditions in which development occurs
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The range of phenotypes produced by a genotype in different environments (in this case,
plant height) is called the norm of reaction
For most of the characteristics discussed so far, the effect of the environment on the phenotype has been slight

Mendel’s peas with genotype yy, for example, developed yellow endosperm regardless of the environment in
which
they were raised
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For other phenotypes, however, environmental effects play a
more important role
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For example,
the himalayan allele in rabbits produces dark fur at the extremities of the body—on the nose, ears, and feet
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The expression of the himalayan allele is thus temperature dependent—an enzyme
necessary for the production of dark pigment is inactivated at higher temperatures
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The himalayan allele is
an example of a temperature-sensitive allele, an allele whose product is functional only at certain temperatures
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Results of a study of
norms of reaction of vegetative offspring of AchUlea, grown at low, medium and high elevations
are shown in Figure 1
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Many plants were collected and three cuttings from each of seven
plants, were grown one each at low elevation (30 metres above sea level), medium elevation
(1400 metres) and high elevation (3050 metres)
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3
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Fig
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3
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In the above discussion, our emphasis has been on the effect of environment in determining the
phenotype due to a specific genotype
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g
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This uncontrolled variation in phenotype may be due to differences in local or internal
environment during development, and is described as developmental noise
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The first condition for the study of effect of different environments on a genotype
would be to have individuals which have same genotype
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A clone is the progeny of a single plant reproduced asexually e
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bacterial
population derived from a single cell
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Similarly inbred lines are obtained in cross-fertilizing individuals
due to fertilization among closely related lines
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For a particular genotype, one could tabulate
the different phenotypes, resulting due to different environments, in which the genotype grows
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This tabulated set of phenotypes will represent the norm of reaction of the genotype
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For example, in Drosophila melanogaster, the normal (natural or wild) body colour is brown and
a hereditary variant has yellow colour, when the larvae of wild type Drosophila with brown body
colour are raised on food containing silver salts, they develop into yellow bodied flies

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