Final answer:
Mendel's conclusion that was NOT based on his monohybrid crosses is that genes assort independently during inheritance, which is from his dihybrid cross experiments.
Step-by-step explanation:
One of Gregor Mendel's conclusions that was NOT based on his monohybrid crosses is d) Genes assort independently during inheritance. This conclusion is actually derived from Mendel's dihybrid crosses, not monohybrid. To recap Mendel's monohybrid cross findings: a) Traits are controlled by discrete units known as genes; b) Alleles segregate during gamete formation, meaning each gamete receives only one allele of a gene; and c) Dominant traits mask the expression of recessive traits. Mendel's law of independent assortment came from studying dihybrid crosses, which involve two sets of traits and demonstrates that the alleles of different genes are passed to offspring independently of each other.