Just for clarification’s sake, are you saying people who were adopted have no ethnicity unless or until they choose ethnicities they like? 
That’s what I’m not getting, what I keep asking about. Why was that information suppressed along the way? Don’t parents inform their children? Mine did, and so on back.
Mine didn’t. Their parents disapproved of their marriage for religious reasons, so my parents never talked about “grandma and grandpa”. Pretty simple.
My family comes from the East End of London, which has traditionally been a bit of a melting pot. We only found out how mixed when my sister did some ancestry research; we are from Suffolk, and from Scotland, and from Belgium, and even from Trinidad. The younger generations of my extended family are even more mixed up. Ancestry isn’t really something I’m hung up on, although it is interesting in an abstract sort of way.
In the context being discussed, ethnicity is based on ancestry. That is exactly what people will answer in the U.S. when asked about their ethnicity. That’s why there are so few who identify as just “American.” Those are the people who don’t know their ancestry. Far more people are just American culturally.
If I were asked about my ethnicity, I would say Scots-Irish. But only because I researched my last name and who settled in my area historically. Not because there is anything remotely Scottish about the way I live my life.
Still, in America, if we can, we like to say we’re from somewhere, no matter how small the connection. Culturally, I just have more in common with Upper Appalachia. We’re mountain folk.
People who are adopted get their ethnicities the same way that people who aren’t adopted get their ethnicities—through how they were raised. That is, not from their genes.
If your (adoptive) parents raised you as an American with some Irish-American traditions then you are ethnically American or Irish-American, regardless of your biological parentage.
Because … say it with me …
Ethnicity is not primarily genetic.
I loved the one with Olivia Colman! She had an Indian g-grandmother (3x IIRC). Fascinating story.
Both my maternal grandparents emigrated from Hungary to the US (separately, my grandmother as a child; my grandfather as a young man), so I knew my ethnicity fairly well on that side. All my dad would say was mostly Scottish, some English on his side. Doing some research I found that my most recent ancestor from outside the US was 1840, so I’m not surprised the details were lost. Turns out I have a small thread of German, and a little bit of Irish (both “types” of Irish). Thought I’d found French ancestry but turned out to be a group of Swiss Anabaptists. But the majority of my ancestors on his side were Scottish.
My parents never suppressed anything it just wasn’t discussed because I was never interested enough to ask. It is entirely possible that they did mention something about it or would have been happy to but…it just held no interest at all.
As has been said, I don’t think it actually matters. Knowing that there was a specific racial or ethnic or regional component to my background can’t possibly make any difference to who I am or the life I lead.
No, that ethnicity is cultural. Your ethnicity is the culture you identify with and others identify you with. Sometimes there can be confusion: what, you’re German? But you’re black! Yes, but I was born in Germany, I’ve got German nationality, the only language I speak fluently is German (English too but not as comfortably), I’m a cop… it’s possible to look more German, but not possible to be more German!
My genetics include French, German and Italian, but the immense majority is Spanish (from different regions) and Basque (from both sides of the border). I’m Italian in a love of their food but then, so’s every single Spaniard :p. About as German and French as my made-in-China pencil. For a foreigner, I’m a Spaniard; to a Spaniard, I’m Navarrese; to a Navarrese or a Basque (from either side of the border), I’m from Pamplona the whole life, grew up in another town and live in a third. “Spaniard” isn’t merely my nationality, it’s my national identity, it’s a culture I identify with as well as having gotten saddled with it at birth; it is perfectly possible for someone to have a nationality which does not correspond to what they identify as (see: DACA; my Italian relatives who kept the nationality because it freed them from Spanish military service) or even to have no nationality (doesn’t mean they don’t have an ethnicity). The level of detail I give to a local provides cultural details that a non-Basque wouldn’t know what to do with.
Some posts here seem to imply that being concerned about ethnicity was something that, if it was thought about at all, died out a generation or two ago, while current generations don’t think about it at all. Exactly the opposite is true in my family. Being interested in our ethnic ancestry is a matter of nerdiness, not pride in ancestry or anything remotely racist. By the 1950’s, which is the earliest that I or my siblings can remember, our parents and grandparents didn’t talk about it very much. My brother and I had to push our grandparents to learn the names (including maiden names) and national origins of our great-grandparents. One of our great-grandparents immigrated to the U.S. in the late nineteenth century, while the others immigrated a generation or two before that.
The difference between my generation and previous generations on this matter was wanting to keep better records. Most of our generation is college-educated, while our parents’ generation didn’t (with a couple of exceptions) go beyond high school, and my grandparents’ generation didn’t even go to high school. Although those generations knew vaguely about their ancestry, they didn’t write anything down. My brother, on the other hand, has done a lot of Internet research to push back our knowledge of our ancestry several more generations. None of this stops us from interracial and inter-ethnic marriages. On the contrary, I think we like the idea of future generations having ancestries that are a global hodge-podge.