A gene is a strand of DNA that is a part of vary long and compacted string of DNA called a Chromosome.
An important reference point along this
compacted string(Chromosome) is called Centromere.
The distance from a gene to the centromere is referred to as the gene’s locus or map location.
is one of a series of different forms of a gene.
“ Polymorphisms are DNA sequences
that differ from the sequences of a majority of a population but are still shared by a certain percentage”.
Polymorphic DNA in humans is great
due to the relatively large size of our genome.
98% of which does not code for genes.
It is estimated that genome sequences
differ by one nucleotide every 1000–1500 bases.
These single nucleotide differences, or single nucleotide polymorphisms (SNPs), may occur in gene-coding regions as well as intergenic sequences.
Polymorphisms are inherited in a mendelian fashion, and locations of many polymorphisms in the genome are known. Therefore, polymorphisms can be used as landmarks, or markers, in the genome to determine the location of other genes.
Polymorphism Structure Detection Method RFLP One or more nucleotide changes that affect the size of restriction enzyme products Southern blot VNTR Repeats of 10–50 base sequences in tandem Southern blot, PCR STR Repeats of 1–10 base sequences in tandem PCR SNP Alterations of a single nucleotide Sequencing, other
Repeats of 1–10 base
sequences in tandem.
AGCT . Occasionally, STRs contain
repeat units with altered sequences.
Microvariants repeat units missing one or more bases of the repeat.
In contrast to VNTRs, the smaller STRs are efficiently amplified by PCR .
The amount of specimen required for STR analysis by PCR is reduced from
1 μg to 10 ng , a key factor for forensic analysis.
STRs within genes are designated according to the gene name.
for example , TH01 is in intron 1 of the human tyrosine hydroxylase gene on chromosome 11, and TPOX is in
intron 10 of the human thyroid peroxidase gene on chromosome 2.
STR alleles are identified by PCR product
size. Primers are designed to produce
amplicons 100–400 bp in which the
STRs are embedded.
The sizes of the PCR products influenced by the number of embedded repeats.
If one of each primer pair is labeled
with a fluorescent marker, the PCR product can be analyzed in fluorescent detection systems . Silver-stained gels may also be used; however, capillary electrophoresis with fluorescent dyes is the preferable method, especially for high throughput requirements.
DNA testing results in peak or band patterns that must be converted to genotype (allele identification) for the comparison of result between laboratories
STR locus genotype is defined by number of repeats in the alleles.
Microvariants alleles containing partial repeats units are indicated by the number of complete repeats followed by decimal points and then the number of bases in the partial repeats.
Y-STR are represented only once per genome and only in the male.
A set of Y-STR alleles comprises a haplotype.
because the Y chromosome cannot change the information (recombine) with another Y chromosome.
Thus ,marker alleles on the Y chromosome are inherited from generation to generation in a single block.
Provide marker loci for Y chromosome, or surname, tests to determine ancestry.
Because the location of many STRs in the genome are known, these structure can be used to map gene. especially those genes associated with disease.
Three basic approaches are used to map genes,
Quality assurance for surgical section using STR.
Proper identification and assurance of uncontamination.
Confirmation of the tissue origin by multiplex PCR