Frequent twins mystery in Brazil solved

An article  in the New York Times highlights the discovery of a gene which causes more twin births. For decades, the people of San Pedro, a tiny town in Candido Godoi, Brazil have produced am inordinate number of twins: 10% of their births were twins compared to 1% in Brazil overall. The researchers studied bapitism certificates, tested the water supply, and did genetic testing. The scientists believe that a small settlement of German immigrants brought a specific gene into the area that gave them a predisposition to having twins. The immigrants bred amongst each other and in a small population, this includes high degrees of inbreeding. The gene is not universal and is not what causes twins in other populations. It is specific to this region.
I would like to know where this gene came from. Why don't we see it in an original population in Germany? Is it only expressed because of the inbreeding? Was it a mutation that occurred before or after settlement?

 twins napping.