#STRs

Molecular Marker

STRs used in parentage testing differ based on:

Short Tandem Repeats exhibit polymorphism due to variation in number of tandemly repeated core units at a locus. DNA polymerase slippage adds or deletes repeat units during replication, generating alleles differing in length but not necessarily base composition, methylation state or expression level. Parentage testing exploits highly polymorphic allele length variation; child inherits one allele length from each parent. Base composition and epigenetic modifications do not determine STR genotype. Counting repeat number provides quantifiable allelic diversity crucial for forensic identity and kinship resolution.

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

In the pedigree showing STRs A-B-C on X-chromosome, recombination is found in:

X-chromosome STRs A-B-C are transmitted as haplotypes with limited recombination due to hemizygosity in males and sequential inheritance. In pedigree analysis, deviation from expected maternal transmission indicates crossover between markers. When offspring III-2 and III-4 display new allelic combinations of A-B-C not present in either maternal chromosome, recombination between those loci must have occurred during maternal meiosis I or II. Other individuals retain parental haplotypes unchanged. Detecting such recombination events allows estimation of genetic distance and linkage analysis, demonstrating meiotic exchange on X chromosome during gamete formation in pedigree.

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 through polymorphic length variation.

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

In STRs, what varies among individuals?

Short tandem repeats comprise tandem arrays of two to six base pair motifs where individual variation derives from different copy numbers of the repeat unit at a given locus. DNA polymerase slippage during replication expands or contracts repeat tracts, creating alleles differing in length but not in base composition or methylation pattern. Enzyme activity is unrelated to structural polymorphism. Consequently, genotyping relies on measuring fragment length variation through capillary electrophoresis, reflecting inherited differences in repeat number critical for forensics and paternity testing.

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

The most polymorphic marker system is:

Simple sequence repeats exhibit exceptional polymorphism due to high mutation rates caused by slipped-strand mispairing during DNA replication. Variable repeat numbers generate many alleles per locus, often exceeding ten in diverse germplasm, providing high polymorphic information content and heterozygosity values. Their codominant, multiallelic nature surpasses biallelic SNPs, low polymorphism RFLPs and dominant RAPD markers that detect fewer loci. Widespread genomic distribution, excellent reproducibility and ease of PCR detection further enhance utility for linkage mapping, population structure and fingerprinting studies in crop plants.

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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