Variation and Polymorphism in DNA


bp, Base pair; kb, kilobase pair; Mb, megabase pair.



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Figure 4-1 Three polymorphisms in genomic DNA from the segment of the human genome reference assembly shown at the top (see also Fig. 2-6). The single nucleotide polymorphism (SNP) at position 8 has two alleles, one with a T (corresponding to the reference sequence) and one with a C. There are two indels in this region. At indel A, allele 2 has an insertion of a G between positions 11 and 12 in the reference sequence (allele 1). At indel B, allele 2 has a 2-bp deletion of positions 5 and 6 in the reference sequence.

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Figure 4-2 Examples of polymorphism in the human genome larger than SNPs. Clockwise from upper right: The microsatellite locus has three alleles, with four, five, or six copies of a CAA trinucleotide repeat. The inversion polymorphism has two alleles corresponding to the two orientations (indicated by the arrows) of the genomic segment shown in green; such inversions can involve regions up to many megabases of DNA. Copy number variants involve deletion or duplication of hundreds of kilobase pairs to over a megabase of genomic DNA. In the example shown, allele 1 contains a single copy, whereas allele 2 contains three copies of the chromosomal segment containing the F and G genes; other possible alleles with zero, two, four, or more copies of F and G are not shown. The mobile element insertion polymorphism has two alleles, one with and one without insertion of an approximately 6 kb LINE repeated retroelement; the insertion of the mobile element changes the spacing between the two genes and may alter gene expression in the region.


Single Nucleotide Polymorphisms



The significance for health of the vast majority of SNPs is unknown and is the subject of ongoing research. The fact that SNPs are common does not mean that they are without effect on health or longevity. What it does mean is that any effect of common SNPs is likely to involve a relatively subtle altering of disease susceptibility rather than a direct cause of serious illness.



Insertion-Deletion Polymorphisms




Microsatellite Polymorphisms



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Figure 4-3 A schematic of a hypothetical microsatellite marker in human DNA. The different-sized alleles (numbered 1 to 7) correspond to fragments of genomic DNA containing different numbers of copies of a microsatellite repeat, and their relative lengths are determined by separating them by gel electrophoresis. The shortest allele (allele 1) migrates toward the bottom of the gel, whereas the longest allele (allele 7) remains closest to the top. Left, For this multiallelic microsatellite, each of the six unrelated individuals has two different alleles. Right, Within a family, the inheritance of alleles can be followed from each parent to each of the three children.

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Nov 27, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Variation and Polymorphism in DNA
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