Molecular marker and their types
Generally molecular marker, Dna marker and genetic markers are different terms. To understand this terms come to following concept :
Genetic marker
A gene or DNA sequence having a known location on a chromosome and associated with a particular trait or gene is used as a genetic marker.
📍Genes were the first markers to be used to prepare the first genetic maps of fruit fly.
📍Genetic markers used in genetics and plant breeding can be classified into two categories:
- Classical markers
- DNA markers
↪️Classical markers
- Morphological markers
- Cytological markers
- Biochemical markers.
Morphological markers also called as visible markers.
📍Morphological Markers usually visually characterized phenotypic traits or characters such as :
🔸flower color 🔸seed shape 🔸growth habits 🔸pigmentation.🔸Fruit Shape 🔸Stem Length 🔸Leaf Shape
📍The way Gregor Johann Mendel used seven contrasting traits. We Can identify morphological markers on the basis of the same traits i.e 1. Stem height 2. Seed color 3. Seed shape 4. Pod colour 5. Pod Shape 6. Flower Position 7. Flower colour etc.
📍However, morphological markers are very limited, and many of these markers are not associated with important economic traits (e.g. yield and quality).
📍These markers are also influenced by environmental factors or the developmental stages.
🏷 Cytological markers
Cytological markers are the unique structural features of chromosomes such as : 1. bands ( G band, C band, Q band, T band ) 2. secondary constrictions.
📍From the banding patterns, displayed in color, width, order and position, Show the difference in distributions of euchromatin and heterochromatin.
📍These chromosome features ( bands and secondary constriction ) are used for :
- Characterization of normal chromosomes
- Detection of chromosomal mutation
- Widely used in mapping and linkage group identification.
📍However, direct use of cytological markers has been very limited in genetic mapping.
🏷 Biochemical markers
BM are gene products that can be detected easily by electrophoresis and specific staining.
📍Enzyme variants such as Isozymes and Allozymes are commonly used as biochemical markers.
[ Allozymes : Enzymes encoded by different alleles of a gene but have the same catalytic activity or function. Allozymes can be separated by electrophoresis and other separating techniques on the basis of differences in molecular size, shape and electrical charge.
Isozymes : Different from allozymes. Isozymes are enzymes that perform the same catalytic function, but are encoded by different nonallelic genes located at different loci ]
📍Biochemical markers are also called as protein markers
📍The major disadvantages of biochemical markers are that they are limited in number.
↪️ DNA marker / Molecular Marker
A particular segment of DNA that is representative of the differences at the genome level.
Or
A molecular marker is a DNA sequence which is easily detected and whose inheritance can easily be monitored ( i.e the inheritance pattern of that marker or Dna sequence means how that Dna sequence [ marker ] evolved or will evolve )
📍DNA marker is also called as molecular marker.
📍Strictly speaking, protein markers and DNA markers are both molecular markers, but the current uses of the term molecular marker is limited to DNA markers.
📍They are identifiable DNA sequences, found at specific locations of the genome, and transmitted by the standard laws of inheritance from one generation to the next.
📍The use of molecular markers is based on naturally occurring DNA polymorphism, which forms the basis for designing strategies to exploit for applied purposes.
📍The first such DNA markers to be utilized were fragments produced by restriction digestion—the restriction fragment length polymorphism (RFLP) based genetic markers.
📍Markers are used to detect polymorphism
[Note : About Dna polymorphism
🔺DNA polymorphisms are produced by changes in the nucleotide sequence or length
🔺Polymorphism involves one of two or more variants of a particular DNA sequence.
🔺 All markers are types of Dna polymorphism
🔺Dna polymorphism is a sequence difference compared to reference standard i.e present in at least 1-2% of a population
You can understand from below image
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| ©researchgate |
Single nucleotide change so polymorphism occur i.e many forms of that part.
↪️ Criteria Of Ideal DNA marker
- High level of polymorphism ( marker must be polymorphic i.e it must exist in different forms so that chromosome carrying the mutant gene can be distinguished from the chromosome with the normal gene by a marker it also caries )
- Even distribution across the whole genome
- Provide adequate resolution of genetic differences
- Co - dominance in expression (so that heterozygotes can be distinguished from homozygotes)
- Have linkage to distinct phenotypes,
- Genome - specific in nature.
- RAPD
- AFLP
- SSLP
- RAPD
- AFLP
- SSLP's
- SNP
- RFLP
- SSR
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