This study was carried out to determine stage progression and specific plant and fruit characteristics in purposefully created summer squash genotypes. In this study, which was carried out in 2019, 59 squash genotypes were assessed with 24 morphological measurements and observations involving plant and fruit characteristics, and one-generation advancement was achieved. Cluster and PCA were implemented to determine the relationship between genotypes determined at the end of single plant selection. A dendrogram was constructed to assess the morphological similarities between the genotypes. In this respect, four main groups and 12 subgroups were determined in the cluster analysis performed among squash genotypes based on 24 morphological variables, while the 10 PC axis explained 94.3% of the total variation in the PCA. When the variance values of the principal component axis were examined, it was seen that the first principal component axis explained 28.8% of the total variation, the second principal component axis explained 26% of the variation, and the third principal component axis explained 11.2% of the total variation. Principal component analyses revealed that (i) 66% of the qualitative (neck in unripe fruit, curving of the neck in unripe fruit, mottling in unripe fruit, and type of mottling in unripe fruit) variation was explained by the first three components. At the end of the study, the phenotypic diversity that exists in this core collection provides valuable information to improve agronomic traits in the summer squash breeding program.