The concept of ethnicity is actually much more fluid and less rigid than 'similar genetic makeup'. The emphasis on the shared genetics within an ethnic group, whether accurate or not, has only become a marker recently. Ethnicity has been a concept for much longer than modern science. Rather than genetics, ethnicity is instead marked by a set of criteria that vary from group to group.

At its most basic, an ethnic group is an in-group of people that defines itself in opposition to an out-group (Barth, 1969: 9). This definition is, however, severely lacking in nuance, and ignores many social factors that impact a group's self-definition. Similarly, this idea ignores sub-groups within an ethnic group, and is open-ended enough to include groups that would not normally be classed as ethnic groups, such as fans of different football clubs. Indeed, this definition is severely lacking as the "instinct to behave in terms of group membership... is basic to most human beings" (Hall, 2002: 11).

There are two ways of thinking about ethnicity beyond this basic definition, the Primordialists and the Instrumentalists. Primordialists believe that ethnicity is a natural, universal phenomenon and that it is one's culture that defines one's ethnicity. The primary markers of 'culture' in this case are language, religion, and physical attributes (not just skin colour, but a variety of attributes deemed important by the ethnic group in question, such as hair length or colour, which can be determined by cultural influences). The biggest issue with this stance is that these criteria are shared beyond individual ethnic boundaries without the people sharing such traits with one another feeling any such ethnic attachment (McInnerney, 2001: 55). For example, the Serbs and Croats of Yugoslavia shared both language and physical attributes, yet were divided on religious grounds. Similarly, the population exchange between Greece and Turkey in 1923 was based upon individuals' religious affiliation, ignoring language and appearance (Hall, 1997: 22-3).

Thus, we can see (and common sense will tell us) that these cultural criteria are not what defines an ethnic group, only what distinguishes it from another. For these criteria to distinguish groups, there must be another unifying idea that cements a group's ethnic consciousness. The belief in a myth of common descent and/or an association with a specific territory, whether that is the current location the group occupies or another location that the group associates within its collective memory, such as an 'ancestral homeland', is now largely considered that cement (Hall, 1997: 25). The emphasis on this concept, however, should be on myth. For, while members of an ethnic group can share genetic traits, such genetic traits are not universal to the group's identity, nor are they largely recognisable to members of the ethnic group. Take the English ethnicity, for example. Most people who consider themselves English will likely refer to the arrival of the 'Anglo-Saxons' as the origin of their ethnic group. However, the groups who were arriving in Britain in the fourth and fifth centuries were not a single ethnic group. Not only does the original myth refer to Angles, Saxons, and Jutes, but modern scholars now consider many of the new arrivals to have included people from other places and ethnic groups, such as Norway, Frisia, and Frankia. Moreover, many Romano-Britons likely assimilated with these incoming groups. As Fleming says, "it would be unwise to see specific regions of the fifth century as impermeable enclaves of particular Continental 'tribes'" (2010: 49), and instead "there were dozens of subtly different and idiosyncratic little societies", possibly as close as the next valley from one another (2010: 50). This isn't even factoring in the arrival of Scandinavians and Normans in later centuries. This myth of shared descent, therefore, is not an immutable fact, rather it is better to imagine it as a fluid construction defined by the context within which it is created, one that must be agreed upon by the group. An excellent book on the immutability of such cultural identifiers is The Invention of Tradition, edited by Hobsbawm and Ranger.

This leads us to the Instrumentalists' position. To put it simply, Instrumentalists see ethnic identity as a "flexible strategy that is highly dependent on the contemporaneous context" (Siapkas, 2003: 175). The strategy, according to Instrumentalists, is a political one, one determined by interactions between competing groups (Barth, 1969: 10). The fact that members of an ethnic group maintain their ethnic consciousness, even when it is of no benefit to them, is a clear flaw in the Instrumentalists' position (Luraghi, 2008: 11).

It is probably best to view the academic conception of ethnicity as a spectrum between these two positions. I, personally, lean towards the Instrumentalist side of the spectrum, but I am very conscious of attributing politics as the sole motivation behind an ethnogenesis. Instead, I think it is a much more natural process (although not an inherent, primordial one) that ethnic groups come to exist, one which can and does include political affiliation. You ask about a theoretical 'homogenized ethnicity' in the US in the future. I would argue there is already one, one based on the national status of 'American' and a common myth of descent of America being a nation of immigrants. The geographical exclusion of Native Americans to reservations is a particularly crude method of ethnic exclusion. There being sub-groups within this ethnic group, such as Italian-Americans, Irish-Americans, and Japanese-Americans, does not diminish the existence of this wider group.

To sum up, while genetic similarities can occur within ethnic groups, there are far more criteria that are necessary for defining an ethnic group. Indeed, genetic similarities are not possible for the everyday person to use to determine someone's ethnicity. When interacting with another person in everyday life, you will base your assumption about their ethnic identity on your perceptions of that person, such as their appearance and their language. Moreover, given the invisibility of genetics, the more visible criteria can actually trump shared genetic material. The idea that shared genetics is the defining characteristic of an ethnic group is dangerously simplifying the concept of ethnicity with roots in racial eugenics. Therefore, when thinking about ethnic identity, you should instead think of what individuals share with one another culturally, traits that manifest themselves much more tangibly in the real world than genetic material.