Final answer:
In a cell with a diploid chromosome number (2n) of 20, there will be 40 chromatids present at mitotic metaphase because each of the 20 chromosomes is replicated into two sister chromatids.
Step-by-step explanation:
The question at hand involves understanding the concepts of ploidy and chromosome numbers. We are given that an organism has a diploid number of chromosomes (2n) of 20 in an ovum, which means that each somatic cell has 20 chromosomes organized in pairs. For such a diploid cell at mitotic metaphase, we anticipate that every chromosome has been replicated and is made up of two identical sister chromatids.
During mitosis, cells duplicate their chromosomes during the S phase of the cell cycle, prior to metaphase. As a result, at metaphase, each of the 20 chromosomes will consist of two sister chromatids, totaling 40 chromatids. Therefore, at mitotic metaphase, the total number of chromatids in a cell with a diploid chromosome number of 20 will be 40.