The late-feathering (LF) gene K on the Z chromosome is an important gene in the chicken industry, which is frequently utilized for the feather sexing, a type of autosexing, of neonatal chicks. The K gene is closely associated with the endogenous ev21 gene from an avian leukosis virus and the incomplete duplication (ID) of prolactin receptor (PRLR) and sperm flagellar protein 2 (SPEF2) genes, and ev21 has been used as a molecular marker to detect LF birds. In the present study, a comprehensive survey for the presence or absence of ev21 and ID across 1,994 birds from 52 chicken breeds, three commercial hybrid groups, and the Red Jungle Fowl revealed that almost all LF breeds have both ev21 and ID. However, only one LF breed (Ingie) has only ID and no ev21. Moreover, this study revealed that almost all early (normal)-feathering (EF) breeds lack both ev21 and ID, but only one breed (White Plymouth Rock) included EF birds with ev21 but no ID. Therefore, regarding LF expression, the results indicated that ID is responsible, but ev21 is not required. Henceforth, ID should be used as a molecular marker to detect LF birds instead of ev21. Because ev21 contains the full genome of an avian leukosis virus, there is a risk of disease development in breeds with this gene. Therefore, the Ingie breed, which has no ev21 at the K locus, represents excellent material for the establishment of new LF stocks.