determining appropriate window size for identifying highly differentiated SNPs
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4.1 years ago
Ana ▴ 200

Hi al, I am trying to identify highly differentiated SNPs via window approach! I have 2 questions: first shall I try sliding window or adjacent windows? I wonder if there is any reference to help me to figure out which window size should I try?

I would appreciate if you know any appropriate reference to point me in that direction! Thanks

window-size genome-scan • 2.9k views
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4.1 years ago

I believe that you are referring to linkage disequilibrium (LD) here.

People generally go for a sliding window of 50 SNPs per window and shifting the window by 5 SNPs each time, and then calculate (per window) the variance inflation factor (VIF) and LD for each batch of SNPs. LD is measured in different ways - take a look here at this Biostars thread: linkage disequilibrium: difference between D' and r-squared

PLINK can be used to measure this for you, assuming your data is in PLINK format: https://www.cog-genomics.org/plink/1.9/ld

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I'm curious why do you favour a sliding window approach, this should lead to a lot of overlapping results wouldn't adjacent windows be more efficient?

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Yes, it generates more information than the adjacent window approach, however, the adjacent window approach has a major flaw in that it only looks at blocks and therefore won't give me information on linkage disequilibrium (LD) between the blocks.

What if I have SNPs ranging from 1-20 and I analyse these in adjacent block sizes of 5 [SNPs]. I will not get information on LD between SNPs 1-5 and those in each other block. If I use a sliding window size of 5 SNPs and shift the window by 1 SNP each time, however, I will get a more continuous feel of LD across these 20 SNPs.

The ideal situation is to actually use a variable-sized sliding window based on SNP density in relation to genomic distance.

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