Abhijit Mitra*, B. R. Yadav, Nazir A.
Ganai and C. R. Balakrishnan
Recent developments in DNA technologies
have made it possible to uncover a large number of genetic polymorphisms at the DNA
sequence level, and to use them as markers
for evaluation of the genetic basis for the observed phenotypic variability. The markers
revealing variations at DNA level are referred to as the molecular markers. Based on
techniques used for detection, these markers are classified into two major categories:
Hybridization-based markers; and PCR-based markers. The molecular markers possess unique
genetic properties and methodological advantages that make them more useful and amenable
for genetic analysis compared to other genetic markers. The possible applications of
molecular markers in livestock improvement have been reviewed with reference to
conventional and transgenic breeding strategies. In conventional breeding strategies,
molecular markers have several short-range or immediate applications (viz. parentage
determination, genetic distance estimation, determination of twin zygosity and
freemartinism, and sexing of pre-implantation embryos and identification of disease
carrier) and long-range applications (viz. gene mapping, and marker-assisted selection).
In transgenic breeding, molecular markers can be used as reference points for
identification, isolation and manipulation of the relevant genes, and for identification
of the animals carrying the transgenes. The progress in development of molecular markers
suggest their potential use for genetic improvement in livestock species.
in recombinant DNA technology and gene cloning during the last two decades has brought in revolutionary changes in the field
of basic as well as of applied genetics by providing several new approaches for genome
analysis with greater genetic resolution. It is now possible to uncover a large number of
genetic polymorphisms at the DNA sequence level, and
to use them as markers for evaluation of the genetic basis for the observed phenotypic
variability. Though theoretically DNA sequencing is the direct approach to reveal such DNA
polymorphism, it has two practical limitations:
(i) sequencing needs initial cloning of the gene or DNA fragment at which allelic
variation of interest exists, and (ii) it requires suitable and cost-effective method for
scoring DNA sequence variation. However, the indirect approach for uncovering of genetic
variation at the DNA level using molecular or DNA markers obviates the above limitations.
Since the first demonstration of DNA-level polymorphism, known as the
restriction fragment length polymorphism (RFLP)1, an almost unlimited number of
molecular markers have accumulated. Currently, more powerful and less laborious techniques
to uncover new types of DNA markers are steadily being introduced. The introduction of
polymerase chain reaction (PCR)2 in conjunction with the constantly increasing
DNA sequence data also represents a milestone in this endeavour. The present review is a
brief account of molecular markers, and their various applications in livestock
Molecular markers and their different
A marker is usually considered as a constituent that determines the function
of a construction (Websters Dictionary). Genetic marker can be defined as any stable
and inherited variation that can be measured or detected by a suitable method, and can be
used subsequently to detect the presence of a specific genotype or phenotype other than
itself, which otherwise is nonmeasurable or very difficult to detect3. Such
variations occurring at different levels, i.e. at the morphological, chromosomal,
biochemical or DNA level can serve as the genetic markers. The markers revealing
variations at the DNA level are referred to as the molecular markers, and on the basis of
techniques used for their detection, these have been classified into two major categories:
Hybridization-based markers, and PCR-based markers.
The hybridization-based markers
These include the traditional RFLP analysis4
as well, wherein appropriately labeled probes for the genes of importance (e.g. cDNA or
genomic DNA sequences) are hybridized on to filter membranes containing restriction enzyme
(RE)-digested DNA, separated by gel electrophoresis and subsequently transferred onto
these filters by Southern blotting. The polymorphisms are then visualized as hybridization
bands. The individuals carrying different allelic variants for a locus will show different
banding patterns. Hybridization can also be carried out with the probes (e.g. genomic or
synthetic oligonucleotide) for the different families of hypervariable repetitive DNA
sequences namely, minisatellite5, simple repeats6, variable number
of tandem repeats (VNTR)7, and microsatellite8 to reveal highly
polymorphic DNA fingerprinting patterns (DFP).
The PCR-based markers
These have, however, removed the necessity of probe-hybridization step, and
have led to the discovery of several useful and easy-to-screen methods. Depending on the
type of primers (i.e. primers of specific sequences targeted to a particular region of a
genome or primers of arbitrary sequences) used for PCR, these markers can be further
sub-divided into the following two groups:
(i) The sequence-targeted PCR assays: In this assay system a particular
fragment of interest is amplified using a pair of sequence-specific primers. In this
category, PCR-RFLP or cleaved amplified polymorphic sequence (CAPS) analysis is a useful
technique for screening of sequence variations that give rise to the polymorphic RE sites
(Figure 1). Such analysis involves amplification of a specific region of DNA encompassing
the polymorphic RE site, and digestion of the amplified DNA fragment with the respective
RE. However, for the screening of the sequence variations that do not lead to creation or
abolition of restriction sites, other approaches namely allele specific PCR (AS-PCR)9,
PCR amplification of specific alleles (PASA)10, allele specific oligonucleotide
(ASO) hybridization assay11, amplification refractory mutation system (ARMS)12,
and oligonucleotide ligation assay (OLA)13 are used. These assays are based on
the principle of high specificity of PCR to selectively amplify specific alleles using
primers that match the nucleotide sequence of one, but mismatch the sequence of other
allele. The sequence-targeted PCR approach is also employed to reveal simple sequence
length polymorphism (SSLP), using a pair of primers that flank the simple sequence repeat
(SSR) motifs (Figure 2). If cloned and sequenced microsatellite loci can be subjected to
PCR amplification and such microsatellite loci can be recovered by PCR, such loci are
termed as sequence tagged microsatellite site (STMS)14 markers. Microsatellite
markers in STMS format can be completely described as information in databases that can
serve as common reference points and will allow the incorporation of any type of physical
mapping data into the evolving map15.
(ii) The arbitrary PCR assays: In this assay system, however unlike the
standard PCR protocol, randomly designed single
primer is used to amplify a set of anonymous polymorphic DNA fragments. It is based on the
principle that when the primer is short (usually 8 to 10 mer), there is a high
probability that priming may take place at several sites in the genome that are located
within amplifiable distance and are in inverted orientation. Polymorphism detected using
this method is called randomly amplified polymorphic DNA (RAPD)16. Based on
this principle, several techniques, which do not require any prior sequence knowledge,
have been developed. However, they differ in number and length of primers used, stringency
of PCR conditions, and the method of fragment separation and detection. In arbitrary
primed PCR (AP-PCR)17, slightly longer primer is used (e.g. universal M13
primer) and amplification products are detected by radiactive or nonradioactive method
following polyacrylamide gel electrophoresis. In DNA amplification fingerprinting (DAF)18
analysis, shorter primer is used (5 to 8 mer) which reveals relatively greater number
of amplification fragments by polyacrylamide gel electrophoresis and silver staining. All
these techniques having similar features can be described by a common term multiple
arbitrary amplicon profiling (MAAP)19. Besides these, a number of modifications
of the basic MAAP assays (namely, template endonuclease cleavage MAAP and RAPD-RFLP) have
been developed as well.
In addition to arbitrary primers, semi-arbitrary primers designed on the
basis of RE sites or sequences that are interspersed in the genome such as repetitive
sequence elements (Alu repeats or SINEs),
microsatellites and transposable elements are also used. In the amplified fragment length
polymorphism (AFLP) assay20, template DNA is digested with two REs, and the
resulting restriction fragments are then ligated with adapters and, subsequently, PCR
amplification is carried out using specially designed primers which comprise (i) a unique
part corresponding to selective bases; and (ii) a common part corresponding to the
adapters and the RE site. Microsatellite-primed PCR (MAP-PCR) assay is carried out using
microsatellite as the primer21.
Properties of molecular markers
In genetic analysis, various types of genetic markers such as morphological,
chromosomal, and biochemical and molecular markers are used. Morphological (e.g. pigmentation or other
features) and chromosomal (e.g. structural or
numerical variations) markers usually show low degree of polymorphism and, hence, are not
very useful for genetic markers. Biochemical markers have been tried out extensively, but
have not been found encouraging as they are often sex limited, age-dependent, and are
significantly influenced by the environment. Sometimes, the various genotypic classes are
indistinguishable at the phenotypic level owing to dominance effect. Furthermore, these
markers reflect variability in their coding sequences that constitute less than 10 per
cent of the total genome. The molecular markers, capable of detecting the genetic
variation at the DNA sequence level, have not only removed these limitations but also
possess unique genetic properties that make them more useful than other genetic markers.
Moreover, they are numerous and distributed ubiquitously throughout the genome. These
follow a typical Mendelian inheritance which usually expresses in a co-dominant fashion,
and are often multiallelic giving mean heterozygosity of more than 70 per cent. They
remain unaffected by the environmental factors, and generally do not have pleiotropic
effects on quantitative trait loci (QTL)22. Since gene expression is not a
prerequisite, virtually the entire genome including the noncoding regions can be
For genetic analysis, molecular markers offer several methodological
advantages that are both attractive as well as amenable. For example: (i) the DNA samples
can not only be isolated very conveniently from blood of live individuals but can also be
isolated from tissues like sperm, hair follicle, and even from archival preparations, (ii)
the DNA samples can be stored for longer periods and can readily be exchanged between the
laboratories, (iii) the analysis of DNA can be carried out at an early age or even at the
embryonic stage, irrespective of the sex, (iv) once the DNA is transferred on to a solid
support, such as filter membranes, it can be repeatedly hybridized with the different
probes, and moreover, heterologous probe and in
vitro-synthesized oligonucleotide probes can also be used, and (v) the PCR-based
methods can be subjected to automation. The properties of different molecular markers are
listed in Table 1.
Applications of molecular markers
Polymorphisms observed at the DNA sequence level have been playing a major role in human genetics for
gene mapping, pre- and post-natal diagnosis of genetic diseases, and anthropological and
molecular evolution studies. Similar approach for exploitation of DNA polymorphism as
genetic markers in the field of animal genetics and breeding has opened many vistas in
livestock improvement programmes23,24. Consequently, enormous interest has been
generated in determining genetic variability at the DNA sequence level of different
livestock species, and in their assessment whether these variations can be exploited
efficiently in conventional as well as in transgenic breeding strategies.
Conventional breeding strategies
Molecular markers can play an important role for livestock improvement
through conventional breeding strategies. The various possible applications of molecular
markers are short-range applications or immediate and long-range applications (Table 2).
Short-range or immediate applications: Molecular
markers have several immediate applications like parentage determination, genetic distance
estimation, determination of twin zygosity and freemartinism, sexing of pre-implantation
embryos and identification of disease carrier. In the following subsections, each of these
applications have been discussed briefly.
(i) Parentage determination: Since the breeding value of an animal is
generally estimated using the information available from its relatives, the knowledge of
correct parentage is therefore a prerequisite. Parentage testing using molecular markers
yields much higher exclusion probability (> 90%) than the testing with blood
groups (7090%) or other biochemical markers (4060%)25. Highly
polymorphic DNA fingerprinting markers26 are quite useful for this purpose.
Recently, DNA fingerprinting with oligoprobes (OAT18 and ONS1) has been successfully used
for determining the parentage of IVF buffalo calf27. With the advent of
PCR-based microsatellite assays, a large number of microsatellite panels have been
reported that are useful for parentage testing in different livestock species. For example
in cattle, Glowatzki-Mullis et al.28 demonstrated
that using two triplex microsatellite co-amplification systems, wrong parentage can be
excluded with almost 99% accuracy. In addition, molecular markers also serve as an useful
tool for animal identification, particularly for verification of the semen used for
(ii) Genetic distance estimation: Genetic distance, a measure of overall
evolutionary divergence, i.e. genetic similarities and dissimilarities between two
populations (such as between species, breeds,
strains), serves as an useful tool for authentication of the pedigree, for
characterization of different breeds or strains within a species, and for evaluation of
the change in variation in species over time. In principle, genetic distance can be
measured on the basis of polymorphic characters occurring at the different levels, viz.
morphological, biochemical, cellular and DNA level. Allelic frequencies of blood groups29
as well as those of other biochemical loci, e.g. serum and milk proteins30,
have been used extensively for the estimation of genetic divergence of different livestock
species. However, a great amount of genetic variations at protein loci remain undetected,
since changes in the underlying nucleotide sequences may not necessarily lead to
corresponding change in the amino acid sequences owing to degeneracy in the genetic code.
Molecular markers capable of generating individual specific DFP patterns, useful for
establishing familial relationships31,32, can serve as an
alternative. The similarities between the DFP patterns that are expressed by band-sharing
values, provide a reliable method for evaluating genetic distance amongst populations33,34.
Presently, the PCR-based RAPD fingerprinting assays are being used for characterization of
zebu cattle breeds35, for detection of genetic variations in cattle and sheep36,
and characterization of highly inbred chicken lines in poultry37.
(iii) Determination of twin zygosity and freemartinism: Correct knowledge of
zygosity of twins, particularly in monotocus animal, is very important. Monozygotic twins
provide means for epidemiological as well as for genetical studies, and also help in
transplant matching. Individual specific DNA fingerprinting techniques have potential
applications in determination of twin zygosity38 and demonstration of
spontaneous XX/XY chimaerism39. Demonstration of XX/XY chimerism in
heterosexual bovine twins, by PCR-RFLP assay using sex-chromosome-specific primers, has
enabled the identification of freemartin animal40,41.
(iv) Sex determination: Sexing of pre-implantation embryos can serve as an
important tool for improving herd for a desired purpose. A large number of invasive and
noninvasive methods for sexing embryos are available. However, ideally the technique to be
applied should not have any adverse effect on embryo survivability, its conception rate
and subsequent development. Besides, the technique should be simple and easy to carry out,
repeatable, and accurate and time saving. Though embryos can be sexed by cytogenetical
method42, this method is quite accurate but invasive and needs a large piece of
embryo. The molecular markers on the other hand, have potential application in
determination of sex of pre-implantation embryos, since the embryos can be sexed using
male-specific or Y-chromosome-specific DNA sequence as probes43. However, this
method is time consuming as well as tedious. Sexing of embryo using PCR-based approach44,45,
involves amplification of male-specific DNA fragment and its visualization in
ethidium-bromide-stained agarose gel following electrophoresis. The PCR-based method of
sex determination offers several advantages over all the other methods: (i) It can be
carried out in less than five hours with almost 100 per cent accuracy44. (ii)
It is less invasive and requires very small quantities (in nanograms) of DNA for PCR
assay, which can be isolated from two to eight cells biopsied from the embryo45.
(iii) It can be done at an early stage of embryo e.g. blastocyst stage (6 to 8 days) or
even earlier at the 1632 cell stage46. (v) The use of multiplex PCR
allows simultaneous genotyping for important loci like milk proteins, diseases carrier,
(v) Identification of disease carrier: Many of the most serious incurable
diseases result not from infections with bacteria or viruses but defects in genomes of the
hosts. Certain allelic variations in the host genome leads to susceptibility or resistance
to a particular disease. Kingsbury47 reported that a particular RFLP in the
Prion protein (Prn P) gene was responsible for the variation in hosts response to
the causative agent, and the incubation time of bovine spongiform encephalopathy (BSE).
DNA polymorphism occurring within a gene helps to understand the molecular mechanism and
genetic control of several genetic and metabolic disorders, and allows the identification
of heterozygous carrier animals which are otherwise phenotypically indistinguishable from
normal individuals. In case of genetic disorders caused by a single point mutation, for
example citrulinaemia48, bovine leukocyte adhesion deficiency (BLAD)49,
and deficiency of uridine monophosphate synthatase (DUMPS)50 in cattle;
hyperkalemic periodic paralysis in horses and malignant hyperthermia in pigs51,
carrier animal possessing the defective recessive allele can be identified easily using
PCR-RFLP assay. Using microsatellite (TGLA116) marker, Georges et al.52 demonstrated the identification
of carrier animals of weaver disease in cattle.
Long-range applications: The
foremost long-range application of molecular markers in conventional breeding includes
mapping of the QTL by linkage. Such mapping information, if available, particularly, for
those loci which affect the performance traits or disease resistance/susceptibility, can
be used in breeding programmes by either within-breed manipulations, like marker-assisted
selection of young sires, or between-breeds introgression programmes.
(i) Gene mapping: Molecular markers have three-fold applications in gene
mapping: (i) A marker allows the direct identification of the gene of interest instead of
the gene product and consequently, it serve as a useful tool for screening somatic cell
hybrids. (ii) Use of several DNA probes and easy-to-screen techniques, a marker also helps
in physical mapping of the genes using in situ
hybridization. (iii) The molecular markers provide sufficient markers for construction of
genetic maps using linkage analysis. Genetic maps are constructed on the basis of two
classes of molecular markers53: Type I markers, that represent the evolutionary
conserved coding sequences (e.g. classical RFLPs and SSLPs), are useful in comparative
mapping strategies where polymorphism is not an essential prerequisite. However, these are
mostly single locus and di-allelic (SLDA) and thus are not useful for linkage analysis. On
the other hand, the type II markers (like microsatellites markers) have higher
polymorphism information content (PIC, a measure of the usefulness of a marker for linkage
studies4) than conventional RFLPs and can be generated very easily and rapidly.
Therefore, major efforts are being made to produce gene maps based on the type II markers.
Further utilization of molecular markers developed from DNA sequences information, namely
ASO and STMS polymorphic markers are also helpful in rapid progress of gene mapping.
(ii) Marker assisted selection: The concept of marker assisted selection
(MAS), utilizing the information of polymorphic loci as an aid to selection, was
introduced as early as in 1900s (ref. 54). However, its application in genetic improvement
of livestock species has been limited due to lack of suitable genetic markers. The
discovery of DNA-level polymorphism in eighties and their subsequent use as molecular
markers has renewed interest in the use of genetic markers in selection of breeding
stocks. Implementation of MAS essentially involves two steps: Identification of the marker
loci that is linked to QTL of economic importance, followed by the utilization of linkage
association in genetic improvement programme. Once linkage between a QTL and a marker
locus is established, it is possible to recognize the alternative QTL allele inherited by
the individual. Such information can then be used for the selection of the breeding stock.
MAS is likely to complement rather than replace the conventional breeding
systems leading to increased rate of genetic improvement through higher selection
intensity, reduction of generation interval and increase in the accuracy of prediction.
Furthermore, selection based on markers is possible in early life or in individuals of
both sexes for sex-limited traits. However, there is a risk of reduced genetic response if
the marker association information is inaccurate, as MAS is a form of indirect selection.
The association between the markers and the QTL is a function of distance between the
markers and target traits, type of linkage phase, and degree of linkage disequilibrium.
Therefore, a high-density gene map with closer linkage is a prerequisite for successful
implementation of MAS55. It is estimated, that an average marker density of
10 cm (520 cm), with about 200250 makers, should be sufficient for
the detection of markerQTL association23. Till recently, gene maps with
average marker interval exceeding 5 cM in pigs56, and 10 cM in cattle57,58,
sheep59 and goat60 were available. However, currently high
resolution maps with 2.5 cM or even less marker density have been published61,62.
One of the best examples of the application of MAS within population is the
selection of young sires before their induction for actual progeny testing63,64.
Inclusion of marker information for selection of young sires in progeny-testing programmes
may lead to an increase of genetic gain by 1530% (ref. 64), and an increase in the
accuracy of prediction65. MAS can be used efficiently across the population
(between breeds) to incorporate the most desirable alleles into a group of selected
individuals/strain/breeds, termed as introgression. Introgression is less common in
domestic animals than in plant species because of limited fertility, longer generation
interval and the greater expense of each individual66; nevertheless,
introgression can be applied in domestic animals for genes with major effects67.
Notable examples are the Booroola gene for increased fecundity66, and genes for
trypanosoma tolerance for NDama cattle68.
Molecular markers are capable of unravelling genetic variations in both the
coding and noncoding sequence regions. Based on this characteristic, Geldermann25
suggested two approaches for the identification of markers that influence QTL, as given
(iia) Polymorphisms in the coding sequences: DNA polymorphisms that occur in
and around the structural and/or regulatory sequences of a gene of physiological
significance (e.g. hormone genes, milk protein genes, MHC) may directly affect gene
expression (by changing the splicing of mRNA, stability of mRNA, rate of gene
transcription or the sequence of gene product) and thereby contribute to the phenotypic
variations among the individuals in terms of productivity and health (disease
resistance/susceptibility). Consequently, such DNA polymorphisms, occurring in the genes
which already have a priori possibility to be
associated or closely linked with the performance trait of importance, can be selected as
Various studies have shown that a number of single point mutations in
structural genes that are inherited in a simple Mendelian manner are associated with
quantitative traits of economic importance. For example, the milk protein polymorphisms
have been found associated with the differences in composition and processing qualities of
milk69, and are linked to some of the production traits70. Most
reports70,71 suggest that as1-casein
A2 and k-casein
B have desirable effects on first-lactation milk yield. Of the many variants of k-casein,
B variant has been found not only to possess some advantageous manufacturing properties
but it also leads to a production of 8 to 10% more cheese60,70. In comparison
to exotic cattle breeds, the frequency of advantageous k-casein
B allele has been found to be low in Indian cattle breeds72. To increase the
frequency of advantageous B allele, the progeny tested sires with favourable genotypes (BB
and AB) can be used in AI programme. Mitra et al.72
reported a new genetic variant (ThrACC ® IleATC
at amino acid position 135) of k-casein
in buffalo. However, its association with milk production traits and processing qualities
are yet to be ascertained.
Allelic variations in either the structural or the regulatory sequences of
growth hormone and prolactin genes have also been studied extensively for their possible
direct or indirect effect on milk production and growth performance. With respect to the Alu I polymorphism in growth hormone gene
(resulting in a leucine (L)\valine (V) substitution at amino acid position 127), the L
allele has been reported to be more frequent among the dairy cattle breeds than the beef
breeds73. In Sahiwal cattle, one of the important milch breeds of India, the L
allele was reported to be more frequent than the V allele74. However according
to Schlee et al.75, this growth
hormone genotype has no significant effect on the breeding values for dairy traits, but
significantly influences breeding value for carcass traits. Beside these, Hoj et al.76 reported association of MspI polymorphism at the third intron of growth
hormone gene with the milk fat yield. However, further study is needed to obtain more
definite conclusions about the effects of DNA polymorphism in growth hormone gene on milk
(iib) Polymorphisms in the noncoding sequences: In this approach, the
variations occurring in noncoding sequences (e.g. flanking regions or intergenic regions)
are utilized indirectly as markers for linkage analysis23,24. Microsatellite
markers, which are often highly polymorphic, are presently being exploited to identify QTL
for economically important traits. Ron et al.77,
using microsatellite markers, identified one marker (D21S4) associated with significant
effects on milk and protein yields. Using 159 microsatellite markers in 14 US Holstein
half-sib families, Georges et al.78
demonstrated the presence of QTL for milk production on five chromosomes (namely
chromosome no. 1, 6, 9, 10 and 20). In another study79, significant association
of microsatellite markers with somatic cell score (SCS, an indicator for susceptibility to
mastitis), productive herd life and milk production traits were established. More
recently, using microsatellite markers Ashwell et
al.80 identified potential QTL for SCS, fat yield, fat percentage, and
protein yield and percentage. Characterization of QTL for economically important traits
using microsatellite markers will help in formulating more efficient breeding programmes
using MAS, especially for bulls prior to progeny testing.
Transgenic breeding strategies
The current livestock breeding strategies largely rely on the principle of
selective breeding. In this method genetic improvement is brought about by increasing the
frequency of advantageous alleles of many loci, though the actual loci are rarely
identified. Moreover, in these methods genes can not be moved from distant sources like
different species or genera due to reproductive barrier. The recent developments in
molecular biology have given rise to a new technology called transgenesis, which has removed the
breeding barrier between different species or genera. Transgenesis has opened up many
vistas in understanding behaviour and expression of a gene. It has also made possible to
alter the gene structure and modify its function81. Of the many applications of
transgenesis, the most convincing one is the development of transgenic dairy animals for
the production of pharmaceutical proteins in milk, and animals with altered milk
The starting point for this technology is the identification of the genes of
interest. In this context, molecular markers can serve as reference points for mapping the
relevant genes that would be the first step towards their identification, isolation,
cloning (positional cloning), and their manipulation. After successful production of
transgenic animals, appropriate breeding methods could be followed for multiplication of
transgenic herd/flock. Molecular markers can also be used for identification of the
animals carrying the transgenes. Though most of the QTL are polygenic in nature and in
transgenesis presently single gene traits are being manipulated82, the
technology nevertheless holds future promises in moving polygenic QTL across the breeding
barriers of animals.
The genetic improvement of animals is a continuous and complex process. Ever
since the domestication of animals by man, he has always remained busy in improving his
animals. In this pursuit many methods have been developed and tested. In recent years, the
demonstration of genetic polymorphism at the DNA sequence level has provided a large
number of marker techniques with variety of applications. This has, in turn, prompted
further consideration for the potential utility of these markers in animal breeding.
However, utilization of marker-based information for genetic improvement depends on the
choice of an appropriate marker system for a given application. Selection of markers for
different applications are influenced by several factors, viz. the degree of polymorphism
skill or expertise available, possibility of automation, radioisotopes used,
reproducibility of the technique, and finally the cost involved. Presently, the pace of
development of molecular markers is tremendous, and the trend suggests that explosion in
marker development will continue in the near future. It is expected that molecular markers
will serve as a potential tool to geneticists and breeders to evaluate the existing
germplasm, and to manipulate it to create animals as desired and needed by the society.
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