#genetic markers

Plant Breeding 4

Marker-assisted selection avoids:

Phenotypic selection suffers from confounding effects of environmental variation, microclimate heterogeneity, soil fertility gradients, and developmental stage, which obscure genetic differences especially for low heritability traits governed by many QTLs. DNA markers represent fixed sequence differences that are independent of external environment, plant age, and tissue, expressed constitutively in genome regardless of moisture or pathogen pressure. Therefore selection based on marker genotype is unaffected by seasonal fluctuations, field heterogeneity, or inoculum pressure that may cause esc

Ref: Tanksley et al. 1989; Xu Y. Molecular Plant Breeding – MAS avoids environmental influence

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Plant Breeding 4

Co-dominant markers are preferred in MAS because they:

Codominant markers reveal both alleles at locus simultaneously, displaying two distinct bands in heterozygote versus one band in each homozygote on gel or capillary electropherogram. This dosage information is invaluable in marker-assisted breeding because breeder can identify heterozygous carriers Aa that contain one copy of recessive or dominant donor allele while retaining recurrent background, and distinguish from homozygous donor AA that may exhibit drag penalty or lack recurrent adaptation. In backcrossing, BC progeny segregate 1:1 Aa:aa, and only heterozygous carrier advances, allowing

Ref: Collard BCY & Mackill DJ 2008 Marker-assisted breeding review: co-dominant heterozygote detection. Euphytica

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Plant Breeding 4

RAPD markers are generally:

Random Amplified Polymorphic DNA employs single decamer arbitrary primers to amplify random genomic segments without prior sequence information using low stringency PCR annealing at 36°C. Amplified fragments present as bands indicate primer binding sites present in both orientations within amplifiable distance; absence indicates sequence divergence or insertion preventing amplification. Since technique detects presence versus absence of dominant allele but cannot distinguish heterozygous Aa from homozygous dominant AA, both produce band, it behaves as dominant marker with 3:1 segregation. More

Ref: Williams JGK et al. 1990 Nucleic Acids Res – RAPD dominant reproducible issues; Welsh & McClelland 1990

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Plant Breeding 4

SSR markers are:

Simple sequence repeats or microsatellites consist of tandem repeats of 1-6 bp motifs such as ATATAT or CAG repeats, abundant and evenly distributed across eukaryotic genomes. Variation in repeat number creates high polymorphism due to slippage during DNA replication by DNA polymerase. Primers flanking repeat amplify locus via PCR, and alleles differ in fragment length detectable by gel electrophoresis or capillary sequencing with fluorescent labels. Because both alleles at locus amplify and can be visualized simultaneously, SSR distinguishes homozygous from heterozygous state: single band for

Ref: Tautz D. Hypervariability of microsatellites; McCouch et al. 1997 Microsatellite markers in rice – co-dominant nature

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MOLECULAR EVOLUTION

Which marker is best for deep evolutionary relationships?

rRNA genes reflects key principle in quiz on molecular evolution, where evolutionary mechanisms shape genetic variation and adaptation. In this context, rRNA genes aligns with experimental and theoretical evidence from population genetics, behavioral ecology and molecular phylogeny. Textbooks like Campbell Biology, Futuyma Evolution and Hartl Principles illustrate supporting data. Understanding why rRNA genes fits helps integrate natural selection, environment.

Ref: Li, Molecular Evolution, Neutral Theory and Molecular Clocks.

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Molecular Marker

Which of the following are codominant markers?

Marker inheritance mode determines informativeness. Codominant markers allow differentiation of homozygote and heterozygote, displaying both alleles in heterozygote. Simple Sequence Repeats are multiallelic codominant, with each allele visible as distinct band length. Single Nucleotide Polymorphisms are biallelic codominant, heterozygote shows both nucleotides. RAPD, ISSR and AFLP generate dominant profiles where presence denotes dominant allele, absence recessive, preventing heterozygote distinction. RFLP is also codominant but pairing SSR with SNP represents most widely used codominant combi

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Molecular Marker

Which marker is most suitable for large-scale mutation screening?

Large-scale mutation screening requires markers that are abundant, genome-wide, amenable to automation and high-throughput detection. SNPs occur every 300-1000 bp in human genome, represent most frequent variation, and can be assayed via genotyping arrays, TaqMan, sequencing and chip platforms enabling millions of genotypes simultaneously. RAPD, AFLP and RFLP are lower throughput, less reproducible, require gel electrophoresis and difficult to automate for millions of samples. Therefore SNP genotyping supports genome-wide association studies, population screening and pharmacogenomics initiativ

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Molecular Marker

A marker showing one band from one parent is:

Mendelian molecular markers are characterized by expression in heterozygotes. Codominant markers like SSR and SNP reveal both alleles from each parent as distinct bands. Dominant markers such as RAPD or AFLP show presence versus absence; a band present in only one parent and appearing in offspring represents dominant inheritance where heterozygote is indistinguishable from dominant homozygote. Recessive requires two copies for phenotype, epistasis involves interaction between loci. Single band inherited from one parent typifies dominant marker behavior, informative but unable to distinguish zy

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Molecular Marker

Which STR property makes it ideal for forensic testing?

Forensic identification demands markers with many alleles, high heterozygosity, small amplicon size suitable for degraded DNA, and uniform genomic distribution. Short Tandem Repeats fulfill these criteria because replication slippage generates extensive polymorphism, with heterozygosity often exceeding 70 percent. Multiple unrelated individuals rarely share complete STR profiles, giving high power of discrimination and low random match probability. Low mutation rate or stability would reduce informativeness, while low assay cost is beneficial but not primary. High polymorphism ensures uniquene

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Molecular Marker

STRs are primarily used in:

Short Tandem Repeats exhibit 1-6 bp repeat units with high mutation rates due to strand slippage, creating many alleles per locus and high heterozygosity. This hypervariability, combined with codominant inheritance, low DNA requirement, and amenability to multiplex PCR, makes them exceptionally discriminating for individual identification. Forensic genetics, paternity testing and kinship analysis rely on standardized STR panels such as CODIS loci. STRs are not primary tools for DNA synthesis, sequencing chemistry or protein expression; their power lies in resolving identity and relatedness thr

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Molecular Marker

Microsatellites are also known as:

Microsatellites consist of 1-6 base pair motifs tandemly repeated 5-50 times, dispersed throughout eukaryotic genomes. DNA polymerase slippage during replication creates extensive length polymorphism, making them multiallelic and highly informative. In molecular marker terminology, this class is designated as Simple Sequence Repeats (SSR) and Short Tandem Repeats (STR), whereas VNTR refers to minisatellites of 10-100 bp. SNPs involve single-base substitutions and ESTs are partial cDNA sequences. Thus microsatellite nomenclature aligns with STR, widely applied in forensic profiling and genetic

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Molecular Marker

SNPs are:

Single nucleotide polymorphisms represent the most abundant class of genomic variation, occurring every few hundred base pairs across genomes, numbering millions in humans and economically important plants. They are codominant in nature, discriminating homozygous and heterozygous states through allele-specific assays, sequencing or array hybridization platforms. Though individually less polymorphic than multiallelic SSR, their high density, amenability to automation and high-throughput genotyping makes them exceptionally powerful. Their use extends far beyond forensics, encompassing associatio

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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