Question: What two factors might lead to genetic similarity between Ashkenazi Jews and the city's second largest ethnic group?

Knowledge 1: Boston
People of Irish descent form the largest single ethnic group in the city, making up 15.8% of the population, followed by Italians, accounting for 8.3% of the population. People of West Indian and Caribbean ancestry are another sizable group, at 6.0%, about half of whom are of Haitian ancestry. Over 27,000 Chinese Americans made their home in Boston city proper in 2013, and the city hosts a growing Chinatown accommodating heavily traveled Chinese-owned bus lines to and from Chinatown, Manhattan. Some neighborhoods, such as Dorchester, have received an influx of people of Vietnamese ancestry in recent decades. Neighborhoods such as Jamaica Plain and Roslindale have experienced a growing number of Dominican Americans. The city and greater area also has a growing immigrant population of South Asians, including the tenth-largest Indian community in the country. [SEP] Knowledge 2: Ashkenazi Jews
A 2010 study on Jewish ancestry by Atzmon-Ostrer et al. stated "Two major groups were identified by principal component, phylogenetic, and identity by descent (IBD) analysis: Middle Eastern Jews and European/Syrian Jews. The IBD segment sharing and the proximity of European Jews to each other and to southern European populations suggested similar origins for European Jewry and refuted large-scale genetic contributions of Central and Eastern European and Slavic populations to the formation of Ashkenazi Jewry", as both groups – the Middle Eastern Jews and European/Syrian Jews – shared common ancestors in the Middle East about 2500 years ago. The study examines genetic markers spread across the entire genome and shows that the Jewish groups (Ashkenazi and non Ashkenazi) share large swaths of DNA, indicating close relationships and that each of the Jewish groups in the study (Iranian, Iraqi, Syrian, Italian, Turkish, Greek and Ashkenazi) has its own genetic signature but is more closely related to the other Jewish groups than to their fellow non-Jewish countrymen. Atzmon's team found that the SNP markers in genetic segments of 3 million DNA letters or longer were 10 times more likely to be identical among Jews than non-Jews. Results of the analysis also tally with biblical accounts of the fate of the Jews. The study also found that with respect to non-Jewish European groups, the population most closely related to Ashkenazi Jews are modern-day Italians. The study speculated that the genetic-similarity between Ashkenazi Jews and Italians may be due to inter-marriage and conversions in the time of the Roman Empire. It was also found that any two Ashkenazi Jewish participants in the study shared about as much DNA as fourth or fifth cousins.
Knowledge 1: Boston
People of Irish descent form the largest single ethnic group in the city, making up 15.8% of the population, followed by Italians, accounting for 8.3% of the population. People of West Indian and Caribbean ancestry are another sizable group, at 6.0%, about half of whom are of Haitian ancestry. Over 27,000 Chinese Americans made their home in Boston city proper in 2013, and the city hosts a growing Chinatown accommodating heavily traveled Chinese-owned bus lines to and from Chinatown, Manhattan. Some neighborhoods, such as Dorchester, have received an influx of people of Vietnamese ancestry in recent decades. Neighborhoods such as Jamaica Plain and Roslindale have experienced a growing number of Dominican Americans. The city and greater area also has a growing immigrant population of South Asians, including the tenth-largest Indian community in the country. [SEP] Knowledge 2: Ashkenazi Jews
A 2010 study by Bray et al., using SNP microarray techniques and linkage analysis found that when assuming Druze and Palestinian Arab populations to represent the reference to world Jewry ancestor genome, between 35 to 55 percent of the modern Ashkenazi genome can possibly be of European origin, and that European "admixture is considerably higher than previous estimates by studies that used the Y chromosome" with this reference point. Assuming this reference point the linkage disequilibrium in the Ashkenazi Jewish population was interpreted as "matches signs of interbreeding or 'admixture' between Middle Eastern and European populations". On the Bray et al. tree, Ashkenazi Jews were found to be a genetically more divergent population than Russians, Orcadians, French, Basques, Italians, Sardinians and Tuscans. The study also observed that Ashkenazim are more diverse than their Middle Eastern relatives, which was counterintuitive because Ashkenazim are supposed to be a subset, not a superset, of their assumed geographical source population. Bray et al. therefore postulate that these results reflect not the population antiquity but a history of mixing between genetically distinct populations in Europe. However, it's possible that the relaxation of marriage prescription in the ancestors of Ashkenazim that drove their heterozygosity up, while the maintenance of the FBD rule in native Middle Easterners have been keeping their heterozygosity values in check. Ashkenazim distinctiveness as found in the Bray et al. study, therefore, may come from their ethnic endogamy (ethnic inbreeding), which allowed them to "mine" their ancestral gene pool in the context of relative reproductive isolation from European neighbors, and not from clan endogamy (clan inbreeding). Consequently, their higher diversity compared to Middle Easterners stems from the latter's marriage practices, not necessarily from the former's admixture with Europeans.
Knowledge 1: Boston
People of Irish descent form the largest single ethnic group in the city, making up 15.8% of the population, followed by Italians, accounting for 8.3% of the population. People of West Indian and Caribbean ancestry are another sizable group, at 6.0%, about half of whom are of Haitian ancestry. Over 27,000 Chinese Americans made their home in Boston city proper in 2013, and the city hosts a growing Chinatown accommodating heavily traveled Chinese-owned bus lines to and from Chinatown, Manhattan. Some neighborhoods, such as Dorchester, have received an influx of people of Vietnamese ancestry in recent decades. Neighborhoods such as Jamaica Plain and Roslindale have experienced a growing number of Dominican Americans. The city and greater area also has a growing immigrant population of South Asians, including the tenth-largest Indian community in the country. [SEP] Knowledge 2: Ashkenazi Jews
A 2001 study by Nebel et al. showed that both Ashkenazi and Sephardic Jewish populations share the same overall paternal Near Eastern ancestries. In comparison with data available from other relevant populations in the region, Jews were found to be more closely related to groups in the north of the Fertile Crescent. The authors also report on Eu 19 (R1a) chromosomes, which are very frequent in Central and Eastern Europeans (54%–60%) at elevated frequency (12.7%) in Ashkenazi Jews. They hypothesized that the differences among Ashkenazim Jews could reflect low-level gene flow from surrounding European populations and/or genetic drift during isolation. A later 2005 study by Nebel et al., found a similar level of 11.5% of male Ashkenazim belonging to R1a1a (M17+), the dominant Y-chromosome haplogroup in Central and Eastern Europeans.

Answer: inter-marriage and conversions in the time of the Roman Empire