Final answer:
Unit factors, as described by Mendel, are now known as genes, which are the units of heredity that determine an organism's traits.
Step-by-step explanation:
Mendel indicated that traits were made up of unit factors. Today, we call unit factors genes. Mendel's work was groundbreaking because it proposed that these unit factors, now known as genes, are inherited as discrete units rather than blending together, as was the common belief of the time. It is these genes that are passed down from generation to generation, which determine an organism's genotype; the combination of these genes leads to certain phenotypes, or observable traits. While alleles are different versions or variants of a gene, the term 'gene' itself is the broader concept representing the unit of heredity Mendel described.