Modern term for Mendels factor

Genes

Explanation:

Mendel's factors were what he suggested were "hereditary units". We now call these genes. Mendel correctly thought that there were different types of the same factor that could be passed on to offspring, making offspring different (such as a red flower vs a white flower). We now know these factors are genes, and the different "types" are known as alleles.

In Mendel's experiments, differences in factors led to different characteristics in the plants.