21st Century Necromancer

"Huh? The hospital said there's no problem with the DNA comparison results?" Tokairin Yūko exclaimed in surprise upon hearing Misumi Mikoto, her face revealing a puzzled expression, "But that doesn't make sense! If the DNA belongs to Yamada Yonegoro, it means he must have been at the scene, and engaged in sexual activity with the victim.

However, he has a perfect alibi, unless he can fly, and could travel from Setagaya Ward to Sumida Ward within ten minutes, and furthermore commit burglary, become sexually aroused, sexually assault and murder the victim, then fly back to Setagaya Ward. That's impossible even if he really could fly, right?"

Misumi Mikoto nodded her head in distress in response to Tokairin Yūko's analysis, frowning with frustration as she said, "But whether it's the residue extracted from Miyoshi Yoko's fingernails or the bodily fluids left on her body, the DNA extracted is all Yamada Yonegoro's, and the comparison has been repeated twice, with consistent results—there's no possibility of error.

So the DNA must be Yamada Yonegoro's! There must be details we've overlooked in this case; otherwise, such a contradiction simply doesn't make sense!"

"Ugh! Where exactly is the problem?" Tokairin Yūko, equally perplexed, slumped into her chair, pouting at the visibly frustrated Misumi Mikoto, who was messing up her short hair.

"Arnold, could it be that Yamada Yonegoro has a twin brother? Is it his brother who committed the crime, but because they are twins, the DNA is too similar?" Kube Rokuro, somewhat nervously, voiced his speculation as he listened to the discussion between Misumi Mikoto and Tokairin Yūko.

"Rokuro, did you really graduate from medical school?" Misumi Mikoto asked Kube Rokuro, her tone both ghostly and questioning, upon hearing his theory.

"Eh?" Kube Rokuro was momentarily startled by Misumi Mikoto's question, but he still nodded affirmatively, saying, "Although it's not a prestigious school, just a third-rate medical university, I do have a medical degree."

However, after hearing his response, Misumi Mikoto simply shook her head, looking at Kube Rokuro with an expression of disdain, her tone a mixture of disgust and pity, "But why don't you understand even this basic medical knowledge?"

Tokairin Yūko followed up on Misumi Mikoto's comment with a jab at Kube Rokuro, "Did you really graduate all on your own? You didn't actually have to retake exams, did you?"

These words made Kube Rokuro blush and hang his head, suddenly unwilling to speak further.

Seeing Kube Rokuro like this, Misumi Mikoto and Tokairin Yūko exchanged glances, becoming perplexed, as if they might have guessed the truth.

With that thought, Misumi Mikoto and Tokairin Yūko felt it was not right to continue poking at Kube Rokuro's sore spot, so after Misumi Mikoto coughed once, she began to explain, "Although medically speaking, identical twins come from the splitting of a single fertilized egg, and from that perspective, their genome sequences—that is, the DNA sequences on their chromosomes—should be exactly the same, but don't forget, somatic cell mutations are commonly present throughout the human body."

"Exactly, somatic cell mutations!" Tokairin Yūko also nodded, emphasizing this point.

"Somatic mutation?" Kube 'Slacker' Rokuro strained his brain, flipping through his sparse memories from class, finding the term somewhat familiar but unable to recall where he had heard it.

Just as Rokuro was trying to remember, someone from the Nakado Department who had been carrying a cup to get water and appeared to have been listening to their discussion spoke up. Observing Rokuro's baffled look, he said nonchalantly, "So-called somatic mutations are DNA mutations that generally occur non-stop due to environmental radiation, viral invasion, chemical environment, and more importantly, random errors in chromosome replication during the cell division in the body's development process."

After speaking, he patted Rokuro's shoulder twice: "If I remember correctly, this is covered in cell biology. You should pay attention in class because the things teachers go over are useful, especially since you've entered this field."

"I got it." Rokuro's face reddened with the pointed advice from the Nakado Department, but despite the embarrassment, he still asked Misumi Mikoto, "But what does somatic mutation have to do with the DNA of twins?"

By now, Misumi Mikoto and Tokairin Yuko were aware that Rokuro lacked knowledge in this area. Without saying much more, they went on to explain directly, "The rate at which DNA mutates varies greatly between different chromosomes and regions within chromosomes. Some areas mutate very quickly, with frequent mutations during cell division, while others divide very slowly, with a mutation occurring only after many divisions.

The mutation rates of DNA in different tissues also show significant differences. On average, the probability of a somatic mutation occurring in each base of a cell during cell division is 6.0 × 10^−8. Since the human genome has 3 billion bases, multiplying that number yields more than a hundred 'somatic mutations' per cell with each division.

Take cancer cells, for example. Initially benign stem cells or other fast-dividing cells accumulate too many somatic mutations over time, even at critical DNA locations. This accumulation of mutations triggers their transformation into cancer cells."

At this point, Tokairin Yuko looked at Rokuro sternly, emphasizing to him, "So please remember, the vast majority of cancer-causing mutations are a type of somatic mutation." Read exclusive content at My Virtual Library Empire

Watching Tokairin Yuko emphasize to Kube 'Slacker' Rokuro, Misumi Mikoto continued, "Getting back to the topic of twins, two embryos from twins undergo many cell divisions from their separation to the birth of the infant, which naturally results in more somatic mutations, and the locations of these mutations are generally random.

Therefore, the DNA sequence of somatic cells at the same body location can be quite different between twins.

Moreover, the blood DNA sequence is determined by the DNA sequence of bone marrow hematopoietic stem cells, which after living for several years or decades, will produce more diverse somatic mutations."

"So, even if Yamada Yonegoro really had a twin brother, their DNA would still be different, right?" Rokuro finally grasped the key point.

"Exactly, that's right!" Tokairin Yuko nodded vigorously and gave Rokuro's shoulder a hearty slap, "See, you can understand if you listen carefully!"