There was an interesting problem that confused some people for a while many years ago.
A survey of school children in Scotland indicated that a larger than normal percentage of Catholic children had an uncommon type of color blindness.
The obvious question was: What does being Catholic have to do with being color blind? A quick check of statistics from other countries indicated normal percentages for Catholics in other countries. That indicated that it was being Scot and Catholic.
Somebody quickly realized that most Catholics in Scotland were descendants of Irish immigrants. A check of statistics in Ireland indicated that this type of color blindness was extremely rare in Ireland and also rarer than usual among Irish in countries other than Scotland. In spite of the statistics, the phenomenon had to be due to being Irish. Strange?
Then the light dawned in somebody's mind. People with this form of color blindness could not tell the difference between blighted potatoes and normal potatoes. Such people used blighted potatoes as seed for next year's crop, resulting in a disastrous crop the next year. These were the first Irish to emigrate from Ireland due to the famine caused by the potato blight. Most of them went to Scotland, the most convenient country (England & Ireland were at odds even then).
That first wave of Irish emigrants mostly went to Scotland and included almost all of those with this type of color blindness. This resulted in its being rarer than usual in Ireland, and higher than usual among Irish in Scotland. Later waves of Irish emigrants (without color blindness) went to the US and various countries other than Scotland, which already had more than its desired share of Irish.