Why does almost everyone here a Y-chromosome.

[RobertColumbia]

That would be interesting. Such a situation could also possibly arise even if the mutation did not occur in the newly inhabited land. If the mutation only survived in a subsect of the original country, such as a family, it is quite possible that they could have all migrated to the colony....

Yes, though a "colonial" HG would probably never completely ouster all other HG's, since this is not known to have ever happened anywhere. If I were to go looking for New World-unique subtypes of European HG's, I would start with the Amish of the USA, who have mostly resisted mixing over the past few hundred years. Nearly all other European immigrants have eventually mixed around and intermarried to some extent, with second-generation German-Americans marrying just-off-the-boat Irish people and having children who marry into an old Cajun family, etc. etc.

There is an interesting parallel in Historical Linguistics (which has some genetic-like tendencies) where many accents and dialects of USA English have retained vocabulary and grammatical forms that are mostly considered obsolete in the UK. For example, most Americans refer to the season preceding winter as "fall", which is an archaic poetic term in the UK. Similarly, most Americans (outside of Boston and New York City) do pronounce the so-called "postvocalic R" (e.g. "Marrrrk Carrrrterrrr, parrrrk yourrrr carrrr hearrrr in Harrrrvarrrrrd Yarrrrrd forrrrr the fairrrrr") that is more associated with rural farmer accents in England.