Final answer:
Mendel's 'units' that determine traits are known as genes, which are located on chromosomes and come in different forms called alleles. The genes segregate during the formation of gametes and combine during fertilization to determine the traits of the offspring.
Step-by-step explanation:
Gregor Mendel concluded that each pea plant has two units for each trait, and each gamete contains one unit. Mendel's "units" are now called genes. Through his experiments, Mendel established the laws of inheritance, and one of these is the law of segregation. This law states that these genes segregate, or separate, during gamete formation, with each gamete receiving one gene from each pair. These genes are located on chromosomes, and during fertilization, the combination of gametes from each parent restores the two-gene state for each trait in the offspring.
What Mendel referred to as unit factors are the alleles, varying forms of a gene, which determine different traits such as pea color. Each diploid organism, including pea plants, has homologous pairs of chromosomes, carrying these alleles that express the physical characteristics.