They shouldn't be too afraid of having something Dutch in them. (Might make them a bit taller too!)
Poles were never afraid of mixing :)
What are you referring to exactly? The R1a haplogroup in general is not carried by 44% of the Polish population.
R1A1 group is just a single group mentioned in Wiki, for example the central european Slavs (Czechs, Slovaks and Poles) make up a group R1a1a1, Poles after undergouing the mutation L260 aquired a group R1a1a1g2 which is visible also in the Czechs and is believed to be natively Polish.
Essentially Slavs have the same basic grouping which becomes divergent but similar through a series of mutations, it doent take an R1a combination to be a Slav, all it takes is the same root identified in all Slavic peoples, the root itself contains several different but similar, neighbouring genes.
In layman terms, there's a group of genes that underwent specific mutations, together they create a cluster of genes specific to a certain genetic group, Slavs have them and they prove the common genetic ancestry - ie there was a small specific group of peoples genetically outfitted in a similar manner who went on to become Slavs.