Merry and Sherry were confused for each other from the day of their birth until they were children, teenagers, and adults. Merry is mistakenly named Sherry almost half the time when they are separated, and Sherry is mistakenly called Merry just as frequently. Even their parents can tell them apart. Since Merry and Sherry were both born from a single fertilized egg, they are monozygotic, or identical, twins. When the embryo split into two cell masses at an early cleavage stage, both cell masses grew into full embryos normally. On April 7, 1955, one infant was called Merry, and the other Sherry.
“They contain the same genes” is a common explanation given for the essentially identical traits of monozygotic twins, such as Merry and Sherry. Naturally, that is untrue. “Identical twins contain progeny replicas of the same parental genes” is a more accurate statement. However, based on this straightforward colloquialism, the majority of people do. Think that a gene’s offspring clones are identical. If identical twins share the same human genome?
Human life begins with a single cell, a small sphere with a diameter of around 0.1 mm. During fetal development, that single cell gives rise to hundreds of billions of cells. An average-sized adult human has roughly 65 trillion cells (65,000,000,000,000) in total. There are offspring copies of each of the 25,000–35,000 genes in each of these 65 trillion cells, with certain exclusions. Furthermore, the body’s cells are dynamic; in certain tissues, new cells are constantly replacing older ones. For example, the bone marrow cells in a healthy person generate over two million red blood cells every minute. The technique by which these genes are replicated is highly exact, although not every progeny reproduction of a gene in the human body is identical. Each cell division requires the duplication of all 3 x 10^9 nucleotide pairs of DNA that make up the human haploid genome.