the gene f5 encodes for blood coagulation factor v , and the 1691g > a f5 transition in exon 10 causes a substitution ( r506q ) known as factor v leiden . this genetic variant is the most common hereditable risk factor for thromboembolic disease ( kujovich 2011 ) . clinical observations with the 1691a f5 allele include an increased risk of deep vein thrombosis ( koster et al . ; rosendorff and dorfman 2007 ) , and 1691a f5 is also associated with an increased risk for pregnancy loss and possibly other obstetric complications ( ridker et al . the 1691a f5 allele is thought to have originated with a single mutation event which took place 21,00034,000  years ago ( zivelin et al . the 1691a f5 allele appears to be generally confined to indo - europeans : the 1691a allele is very rare or non - existent in asia ( 0.6  % ) and some regions of africa ( 0.0  % ) ( hira et al . 2002 ; nasiruddin et al . in various parts of europe the frequency of this variant has been described as uneven ( rees et al . 2000 ; adler et al . 2010 ) , with high frequencies observed in the czech republic ( 5.1  % , n  =  2819 ) ( prochazka et al . 2003 ) and turkey ( 5.0  % , n  =  2003 ) ( see fig.1 for references).fig . 1 
 a. average allele frequencies for 1691a f5 across europe . a. 1691a f5 frequencies :   > 3  % ,   <3  % ( n.d . = no data ) . slavic data ( adler et al . 2012 ) : poland 
 n  =  1588 , czech republic 
 n  =  2819 , slovakia 
 n  =  152 , slovenia 
 n  =  526 , croatia 
 n  =  749 , serbia / montenegro 
 n  =  499 ; macedonia 
 n  =  130 , bulgaria 
 n  =  506 , russia 
 n  =  539 , ukraine 
 n  =  172 , belarus 
 n  =  80 ; bosnia / herzegovina 
 n  =  67 ( valjevac et al . 2013 ) . other data : group 1 : albania ( n  =  70 ; arsov et al . 2007 ) , france ( n  =  1242 ; leroyer et al . 1997 ; baba - ahmed et al . 2007 ; pasquier et al . 2009 ) , iceland ( n  =  96 ; rees et al . 1995 ) , netherlands ( n  =  2631 ; rosendaal et al . 2006 ) , portugal ( n  =  303 ; ferrer - antunes et al . 1995 ; serrano et al . 2011 ) , spain ( n  =  889 ; garcia - gala et al . 1997 ; aznar et al . 2000 ; frances et al . 2006 ) , switzerland ( n  =  100 ; redondo et al . 2005 ) , germany ( n  =  2199 ; aschka et al . 1996 ; schambeck et al . ; toth et al . 2008 ) , greece ( n  =  932 ; foka et al . 1995 ; lambropoulos et al . 1997 ; antoniadi et al . 1999 ; ioannou et al . 2001 ; shemirani et al . 2011 ) , italy ( n  =  226 ; de stefano et al . 1999 ; sottilotta et al . 2009 ) , norway ( n  =  807 ; ulvik et al . 1998 ; berge et al . et al . 1997 ; kjellberg et al . 2010 ) , turkey ( n  =  2003 ; ozbek and tangun 1996 ; akar et al . 2003 ; agaoglu et al . 2005 ; eroglu et al . 2006 ; ulukus et al . ( n  =  491 ; rees et al . 1995 ; catto et al . 1999 ) . b. significant differences ( p  <  0.05 ) between 1691a f5 allele frequency values in european countries , as analyzed by tukey tests . capital letters indicate countries with high frequency values significantly different from at least one country ; lower case letters indicate which countries had significantly different , and lower , allele frequencies than those with the corresponding capital letter 
 a. average allele frequencies for 1691a f5 across europe . a. 1691a f5 frequencies :   > 3  % ,   <3  % ( n.d . = no data ) . slavic data ( adler et al . 2012 ) : poland 
 n  =  1588 , czech republic 
 n  =  2819 , slovakia 
 n  =  152 , slovenia 
 n  =  526 , croatia 
 n  =  749 , serbia / montenegro 
 n  =  499 ; macedonia 
 n  =  130 , bulgaria 
 n  =  506 , russia 
 n  =  539 , ukraine 
 n  =  172 , belarus 
 n  =  80 ; bosnia / herzegovina 
 n  =  67 ( valjevac et al . 2013 ) . other data : group 1 : albania ( n  =  70 ; arsov et al . 1997 ; baba - ahmed et al . 2007 ; pasquier et al . 2009 ) , iceland ( n  =  96 ; rees et al . 1995 ) , netherlands ( n  =  2631 ; rosendaal et al . 1995 ; heijmans et al . 1998 ; van dunne et al . 2006 ) , portugal ( n  =  303 ; ferrer - antunes et al . 1995 ; serrano et al . 2011 ) , spain ( n  =  889 ; garcia - gala et al . 1997 ; aznar et al . 2006 ) , switzerland ( n  =  100 ; redondo et al . 1999 ) . 2005 ) , germany ( n  =  2199 ; aschka et al . 1996 ; schambeck et al . 1997 ; nabavi et al . 1998 ; ehrenforth et al . 1999 ; lichy et al . 2006 ; toth et al . 2008 ) , greece ( n  =  932 ; foka et al . 2011 ) , italy ( n  =  226 ; de stefano et al . 1999 ; sottilotta et al . 2009 ) , norway ( n  =  807 ; ulvik et al . 1998 ; berge et al . 2007 ) , romania ( n  =  42 ; fischer et al . 2006 ) , sweden ( n  =  1343 ; svensson et al . 1997 ; kjellberg et al . 2010 ) , turkey ( n  =  2003 ; ozbek and tangun 1996 ; akar et al . ( n  =  491 ; rees et al . 1995 ; catto et al . b. significant differences ( p  <  0.05 ) between 1691a f5 allele frequency values in european countries , as analyzed by tukey tests . capital letters indicate countries with high frequency values significantly different from at least one country ; lower case letters indicate which countries had significantly different , and lower , allele frequencies than those with the corresponding capital letter we provide a summary study of the frequency distribution of 1691a f5 across europe by addition of values from 12 countries / regions which are predominantly populated by slavic populations ( note that serbia and montenegro separated in 2006 - data in this article precede this date ) . the databases medline ( u.s . national library of medicine , bethesda , usa ) and several websites providing local language articles ( e.g. , www.google.ba for bosnia / herzegovina , www.google.hr for croatia ; google inc . , mountain view , california , usa ) were searched until march 2012 . keywords and/or mesh terms used were ( together with country / region names ) :  1691a f5  ,  leiden  ,  fv ,  f5  ,  fvl  ,  factor 5  ,  fv  . data for bosnia / herzegovina were obtained from our previous study ( valjevac et al . data were included from : groups of blood donors , those defined as  healthy summary data in europe for nm_000130.4:c.1691g > a f5 is presented as a map . in order to test the significance of the resulting distribution , a chi - squared test for independence ( ulukus et al . 2006 ) and post - hoc pair - wise proportion comparison tests were carried out between all countries by two methods : ( 1 ) as in zar ( 1999 ) : tests analogous to , and refered to here as ,  tukey tests ( critical values from r package dkt ; lau 2011 ) ; ( 2 ) z - tests with bonferroni correction ( microsoft excel 2007 , redmond , wa , usa ; http://www.csun.edu/~vchsc006/469/z.html ) . a map showing average frequencies for 1691a f5 throughout europe is given in fig . comparison of the 1691a f5 allele frequency values in all populations , assessed by the chi - squared test , gave a significant value ( p  <  0.001 ) showing the presence of differences between countries / regions . 1 arbitrarily delimits a group of countries with low frequency ( group 1 ) from those with high frequency ( group 2 ) , with an arbitrary cut - off at 2.8  % . although this was an arbitrary cut - off , with post - hoc tukey and z - tests the two groups of countries / regions were found to have only one intra - group significant difference ( between czech republic , 5.1  % , n  =  2819 ; and denmark , 3.9  % , n  =  20242 ) , but many significant differences between the groups ( fig.1b ) . group 1 consisted of countries / region with 1691a f5 allele frequency < 2.8  % ; the average frequency in the group was 1.9  % . countries in group 1 included : russia , finland , ukraine , poland , slovenia , serbia / montenegro , croatia , the netherlands , france , spain , portugal ( these countries have reasonable sample sizes , and each has at least one significant difference with another country s value ) . if those countries with no significant differences at all ( with smaller sample sizes ) were included , then this group consisted of most ( ten ) slavic populations together with another eight non - slavic countries ( fig.1 ) . group 2 consisted of countries with 1691a f5 allele frequency 2.8  % ; the average frequency in the group was 4.0  % : countries with at least one pair - wise significant difference by tukey tests included : sweden , norway , denmark , germany , u.k . , the czech republic , austria , hungary , italy , bulgaria , greece and turkey . including those countries with no significant differences at all ( with small sample sizes ) , this group consisted of 13 countries including the predominantly slavic countries czech republic and bulgaria ( fig .  1 ) . tukey tests and z - tests gave similar numbers of pair - wise differences between countries ( 66 and 59 , respectively ; tukey test results are shown in fig.1b . ; however , the tukey tests defined statistically significant differences involving slovenia ( 2.5  % , n  =  526 ) , russia ( 2.4  % , n  =  539 ) and serbia and montenegro ( 2.2  % , n  =  499 ) , which were not identified using z - tests , whereas z - tests defined differences involving romania ( 8.3  % , n  =  42 - note the very small sample size ) . additionally the other tukey test differences were more evenly distributed whereas with z - tests most differences were concentrated on spain ( 2.0  % , n  =  889 ) , and finland ( 1.1  % , n  =  1285 ) . in europe the average 1691a f5 allele frequency was found to be 3.5  % ( n  =  44,627 , references in fig.1 ) . in order to highlight the distribution of high frequency populations , european countries / regions were arbitrarily divided into two groups : with low ( group 1 ) or high frequency ( group 2 ) , with an arbitrary boundary at 2.8  % ( delineated by thick lines in fig .  1 ) . the group of high frequency countries ( i.e. , group 2 ) was found to have a north - southeast band distribution . a pocket of low frequency with some southern slavic countries ( slovenia , croatia , bosnia and herzegovina , and serbia and montenegro ) and albania ( although note low sample size ) was found to be enclosed by this band of high frequency . we can therefore surmise that the band of high frequency was created by founder effect seeding , and the pocket of low frequency might have been created with the purported movement of eastern slavic populations with low frequency to this region in the fifth to seventh centuries . however , conclusions referring to the migration of slavic peoples have to take into account the fact that the czech republic and bulgaria have high frequencies of this allele . especially note that the czech republic has the highest frequency value in europe ( 5.1  % ; with a reasonable sample size of 2819 ) but note that this value is not significantly different to values in the neighboring countries germany and austria . additionally , therefore , it could be hypothesized that these result from high influx of peoples carrying the variant from neighboring populations . it is not known to what extent positive or negative selection acts on this allele - it is thought that the negative impact of 1691a f5 before reproductive age exists but reports in children are scanty ( el - karaksy and el - raziky 2011 ) . from the north - southeast band of high frequency of the 1691a f5 allele there seem to be no indications of possible environmental effects which could vary according to location ( and act as selection ) on this allele . although such a map in fig .  1 is informative , the statistical significance of the distribution found depends directly on the sample sizes of the data ( see legend to fig.1 ) . additionally , there are several ways to do post - hoc pair - wise comparison tests between countries , which take into account sample sizes . by using tukey tests or z - tests it was found that there was only one intra - group significant difference ( within group 2 between czech republic and denmark ; fig.1b ) , but many significant differences between groups . ( significance maps at the p  =  0.01 level , for either test , were very similar - not shown . ) this means that inferences from individual country values should be made with caution , and the high frequency group of countries could be treated more or less as one block ( and likewise for the low frequency group ) . 1 represents the present state of knowledge concerning the distribution of the 1691g > a f5 allele , and theoretically increasing the sample sizes would increase the number of pair - wise statistical differences shown in fig.1b . it should be noted , however , that several sample sizes are already reasonably large e.g. , the netherlands ( 2.2  % , n  =  2631 ) , germany ( 3.8  % , n  =  2199 ) but show no intra - group significant differences by either of the methods used . despite the overall similarities in the conclusions drawn from the tukey and z - tests , it should be noted that these analyses suggest ( and confirm ) the appropriateness of tukey tests , rather than z - tests with bonferroni correction , for this type of study . the reason for this is that any inference from differences involving romania with only 42 subjects is rather dubious ( as is the rather high allele sample frequency of 8.3  % , which might not be a good representation of the frequency in the population ) . the z - tests , however , gave six differences , between romania and : finland , poland , ukraine , croatia , spain , and portugal ( data not shown ) , whereas the tukey tests gave no significant differences involving romania ( fig.1b ) . the clinical effects of the 1691a f5 variant are fairly well established , with a three to eight - fold increased thrombotic risk for heterozygotes , and up to 80-fold increased risk for homozygotes , found in a pooled analysis of eight case  control studies with over 2300 cases from western european countries ( emmerich et al . control study ( with 160 cases ) a high frequency of thromboembolism was also found to be associated with the 1691a f5 allele : 21  % of patients with venous thromboembolism carried the 1691a f5 allele , but only 4  % of healthy controls ( coen et al . 2001 ) . further studies could be done to confirm whether or not the presence of the 1691a f5 allele does indeed provide a similar risk for venous thromboembolism across europe . in summary , a north - southeast band distribution of 1691a f5 high frequency in europe was found by the addition of data collated and presented from central , eastern and south - eastern europe . a pocket of low frequency in some southern slavic countries was found to be enclosed by this band of high frequency . the reasons for this distribution are not known , but perhaps reflect founder effects following migration after the last ice age , followed by the migration of slavic peoples from the fifth century . there seem to be no indications of variation in environmental selection due to geographical location .
the allele 1691a f5 , conferring factor v leiden , is a common risk factor in venous thromboembolism . 
 the frequency distribution for this allele in western europe has been well documented ; but here data from central , eastern and south - eastern europe has been included . in order to assess the significance of the collated data , 
 a chi - squared test was applied , and tukey tests and z - tests with bonferroni correction were compared . 
 results : a distribution with a north - southeast band of high frequency of the 1691a f5 allele was discovered with a pocket including some southern slavic populations with low frequency . 
 european countries / regions can be arbitrarily delimited into low ( group 1 , < 2.8  % , mean 1.9  % 1691a f5 allele ) or high ( group 2 , 2.8  % , mean 4.0  % ) frequency groups , with many significant differences between groups , but only one intra - group difference ( the tukey test is suggested to be superior to the z - tests ) . conclusion : in europe a north - southeast band of 1691a f5 high frequency has been found , clarified by inclusion of data from central , eastern and south - eastern europe , which surrounds a pocket of low frequency in the balkans which could possibly be explained by slavic migration . 
 there seem to be no indications of variation in environmental selection due to geographical location .