Hirokazu Kobayashi
CEO, Green Insight Japan, Inc.
Professor Emeritus and Visiting Professor, University of Shizuoka
At the UN Committee on the Elimination of Discrimination against Women in Geneva, Switzerland, the Japanese Imperial Household Law, which stipulates that only male descendants of the Emperor can succeed to the throne, was discussed. On October 14, Ms. Nami Katsuragi, president of the NGO "Association of Citizens to Protect the Imperial Lineage", said in her speech that at the review meeting on October 17, the Japanese government stated that "the Emperor is the priest of ceremonies, and how the imperial throne is inherited is a matter that concerns the very foundations of the nation, so it is inappropriate to address it in light of the Convention on the Elimination of All Forms of Discrimination against Women." Empress Suiko, who ascended to the throne in 593, is well-known for her efforts to establish a centralized state system with the Emperor at its core, alongside figures like Soga no Umako and Prince Umayado (Prince Shōtoku), who sent envoys to the Sui dynasty, established the Twelve Level Cap and Rank System, and drafted the Seventeen-Article Constitution. Empress Suiko is considered Japan’s first female emperor. Including her, there have been ten reigns by eight female emperors throughout history. These include Empresses Suiko, Kōgyoku, Saimei (the same individual as Kōgyoku), Jitō, Genmei, Genshō, Kōken, Shōtoku (the same individual as Kōken), Meishō, and Go-Sakuramachi. Is there a contradiction between the current Imperial Household Law and these historical facts?
The current Emperor, Naruhito, has only one child, Princess Aiko. Still, the Imperial Household Law states: “Article 1: The throne shall be succeeded by a male descendant belonging to the male lineage of the Imperial family.” Therefore, Princess Aiko cannot inherit the throne. Without amending this law, the order of succession is as follows: First in line is Prince Fumihito, the Emperor's younger brother; second is Prince Hisahito, Prince Fumihito’s son; and third is Prince Masahito, the second son of Emperor Shōwa’s younger brother. A female emperor is a woman who inherits the throne from a male emperor. They are considered temporary or interim rulers to maintain the male lineage. In contrast, an emperor from the maternal line implies that the emperor’s lineage is inherited through a woman. According to the Nihon Shoki (Chronicles of Japan), the first Emperor, Jimmu, is said to have ascended the throne in 660 BCE. Counting from then, the current Emperor is the 126th. While there have been female emperors, there has never been an emperor of the maternal line. The criticism from the United Nations Committee on the Elimination of Discrimination against Women is based on the claim that this stems from gender discrimination. On the other hand, the founder of the Imperial House is said to be the Sun Goddess Amaterasu. From a Shintoistic perspective, viewing the Emperor as a “descendant of the deity,” rejecting an empress of the maternal line is similar to restricting the role to males, akin to the Catholic Pope or the Dalai Lama.
Humans have two types of sex chromosomes: X and Y. Human somatic cells consist of 23 pairs of chromosomes inherited from both parents. Among these, XX chromosomes result in a female, while XY chromosomes result in a male. The Y chromosome is inherited exclusively through the paternal line and contains 156 genes. Apart from the sex chromosomes, the 22 pairs of autosomes undergo pairing and crossing-over in the reproductive cells of both males and females, allowing for the exchange of genetic material. After this mixing, one of each chromosome pair is passed down to the offspring. However, the X and Y chromosomes pair only in specific regions within male reproductive cells, known as the pseudoautosomal regions (PAR). These regions are located near the ends of both chromosomes and contain similar genetic information on both the X and Y chromosomes. Chromosome pairing and subsequent crossover can only occur within these PAR regions. The total length of these regions, PAR1 and PAR2, is 3.1 million base pairs (3.1 Mb). The Y chromosome is 51 million bases (51.0 Mb) long, with the PAR regions making up about 6% of its length and coding for 20 genes. Thus, 94% of the Y chromosome does not undergo crossover, although mutations can still occur. The mutation rate of the Y chromosome is higher than that of other chromosomes, estimated to be approximately 1.0 × 10⁻⁹ mutations per base per year. It is believed that historical figures, starting from the 15th Emperor Ōjin, existed, but even the reign of the next emperor, Emperor Nintoku, which spanned from 313 to 399, seems unusually long. A prevalent theory suggests that the first Emperor Jimmu, if he existed, would have lived around 400. Calculating from that time until now—about 1,600 years—results in an estimated 1.0 × 10⁻⁹ × 51 million bases × 1,600 years = 82 mutations. Mutations can involve base sequence substitutions, deletions, or insertions, but 82 mutations across 51 million bases may be considered negligible, depending on the type and location of the mutation. This suggests that, assuming the existence of Emperor Jimmu, the genetic information in the Y chromosome of the current 126 generations has remained largely intact, making it a precious case biologically. If an emperor from the maternal line were to ascend, the lineage of the Y chromosome in the world's longest-reigning Japanese Imperial family would cease. In contrast, human mitochondrial DNA codes for 37 genes and is maternally inherited. While mitochondrial DNA does not pass from males to the next generation, maternal lineage ensures its transmission, albeit with genetic mutations.
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