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Sunday, November 11, 2012

The levels and dating of admixture in Belorusians


Following Dienekes's suggestion on using Pathan and Lithuanian samples as references for ROLLOFF analysis, i decided to undertake a second attempt of formal analysis of admixture and dating of admixture events in Belorusian samples which are available to me: the reference dataset of Belorusians from Behar et al.2011., and Belorusian samples collected by our project.


Below  you can glean the results of experiment which i deem less noisy in contrast to my previous attempt. 

valid snps: 746877
group 0 Lithuanian
group 1 Pathan
number admixed: 13 number of references: 2
numsnps: 746877  numindivs: 55
starting main loop. numsnps: 158101


Summary of fit:

Formula: wcorr ~ (C + A * exp(-m * dist/100))

Parameters:
   Estimate Std. Error t value Pr(>|t|)  
C 2.332e-04  3.029e-04   0.770  0.44165  
A 3.306e-02  1.227e-02   2.695  0.00728 **
m 1.169e+02  3.851e+01   3.037  0.00252 **
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Residual standard error: 0.006508 on 493 degrees of freedom

Number of iterations to convergence: 0
Achieved convergence tolerance: 9.103e-06

mean (generations):  116.9416
jackknife (generations)   105.086+-52.591 
The date of admixture event in Belarusian_V sample with Belarusian and Pathan being reference populations appears to be very close to the date which was estimated by Dienekes for Lithuanian [Lithuanian_D;Pathan].


Inference of Admixture Parameters in Belarusians using Weighted Linkage Disequilibrium

On 1 November 2012 Po-Ru Loh, Mark Lipson, Nick Patterson, Priya Moorjani, Joseph K Pickrell, David Reich, Bonnie Berger announced and published their new paper, in which they introduced a new approach that harnesses the exponential decay of admixture-induced linkage disequilibrium (LD) as a function of genetic distance. They proposed a new weighted LD statistic that can be used to infer mixture proportions as well as dates with fewer constraints on reference populations than previous methods.


I haven't had enough time to investigate this method in full extent, but i used a software package ALDer which implements the weighted LD statistic for a quick & dirty experiment of dating the admixture events in Belarusian sample:

 

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