The phrase "single genus" refers to a group of organisms that belong to the same biological category. In simple terms, it means different species that are closely related and share common characteristics. Full definition
At 2.5 yrs we had an opportunity to participate in the American Gut project which showed her gut was 80 % proteobacteria, and 65 % of her entire gut was a single genus: Stenotrophomonas. (infantreflux.org)
The closest relatives of the genus are also confined to the Australasian region with the exception of a single genus living in South America. (sciencedaily.com)
Thus, a single genus of crinoid is known from the early Triassic, and is ancestral to all «deep water» extant articulate crinoids. (fossilmuseum.net)