Short answers, pulled from the story.
Haplogroup I-Z63, also designated I1a3 by the International Society of Genetic Genealogy, is a Y-chromosome haplogroup most common in England, Scotland, Germany, Fennoscandia, and Poland. Its progenitor is believed to have lived in Jutland around 2500 BCE. The haplogroup is correlated with a genetic marker called DYS456, which typically reads below 15 in carriers.
Haplogroup I-Z63 is believed to have originated in or near what is now Denmark, based on its geographic distribution and the location of the earliest known archaeological sample. YFull estimates it formed approximately 4,600 years ago, around 2600 BCE. The current working hypothesis places the progenitor in Jutland around 2000 BCE.
Chester A. Arthur, the 21st president of the United States, is believed to belong to haplogroup I1-Z63 in the branch BY151 > BY351, based on the Arthur family DNA project. Clan Hamilton of Scotland, William Bradford of the Plymouth Colony, and a warrior buried in Bodzia Cemetery who may have been the Kievan prince Sviatopolk the Accursed are also associated with the haplogroup.
Despite Jutland being the presumed homeland of I-Z63, the haplogroup is nearly absent in modern Denmark. Repeated displacement of the Jutish population by outside groups, including an attack by King Hacon of Norway in 945 that drove survivors far inland, likely scattered the original carriers away from the peninsula.
Three individuals at the Kowalewko burial site in Poland, associated with the Wielbark culture, carried the I-Z63 subclade I-L1237, which Polish researchers identified in 2017. Scholars link the Wielbark culture to the Gothic tribe of late antiquity, making I-L1237 a plausible genetic marker for Gothic ancestry. The Jutish Hypothesis further argues that Gothic and early British settlement patterns reflect the same ancestral migration.
The Bodzia Cemetery warrior, catalogued as VK157 and buried around 1010-1020 AD in Poland, carried haplogroup I-Z63 in the subclade I-S2077. Strontium analysis showed he was not local to the area, and he is believed to have arrived in Poland with Sviatopolk the Accursed, Prince of Kiev. Some researchers speculate VK157 may have been Sviatopolk himself, though the identification remains unconfirmed.