Intersex

An organism that has a distinction between male and female, and whose reproductive organs and secondary sexual characteristics are neither male nor female, but a mixture of both, or the nature of such an organism. Most organisms are infertile or produce only one type of germ cell. Intersex is thought to occur when all the cells in the body have the same genotype, and an individual that develops as one sex at first changes to the other sex due to hormonal abnormalities and remains intermediate between the two sexes. It is also distinguished from hermaphrodites, which normally produce both male and female reproductive organs and germ cells, and sex mosaics, or gynandromorphs, in which some parts of the body are male and other parts are female due to deletion, recombination, or mutation of parts of chromosomes during development. Intersex is divided into female intersex, male intersex, and true intersex, depending on the degree of sex-male sexuality.

Intersex was first observed by R. B. Goldschmidt in 1915, when he crossed Japanese and European species of the gypsy moth, Lymantria dispar , a forest pest, and found various degrees of intersex in the offspring according to a certain rule. He explained that intersex occurs when crossbreeding different species because there is a difference in the strength of the factors that determine sex between the Japanese and European species. Intersex also occurs in fruit flies, where sex is determined by the ratio of the number of sex chromosomes to the number of autosome pairs; a ratio of 2:2 results in normal females, 1:2 results in normal males, and 2:3 results in intersex. It also occurs in livestock such as goats and pigs.

Intersex in humans is called hermaphroditism, and is often caused by abnormalities in the differentiation of the gonads, such as Klinefelter syndrome, in which the testes fail to develop due to a sex chromosome abnormality (XXY, an increase in an inactive X chromosome), Turner syndrome, in which the ovaries fail to develop due to a sex chromosome abnormality (XO, in which there is only one X sex chromosome), and true hermaphroditism, in which both testes and ovaries are present.

[Kuroda Yukiaki]

"Basic Genetics" by Yoshimaro Tanaka (1951, Shokabo) " ▽ "Human Genetics" by Takashi Komai (1966, Baifukan)" ▽ "Introduction to Basic Developmental Biology" by Mamoru Ichikawa, new edition (1968, Shokabo)" ▽ "Genetics, Vol. 1 , by R.H. Tamarin, supervised translation by Motoo Kimura, translated by Ichiro Fukuda et al. (1988, Baifukan)"

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

雌雄の区別のある生物で、その生殖器官や第二次性徴が、雄でもなく雌でもなくその両方が入り混じった中間型となっている個体またはその性質をいう。多くは生殖能力がないか、どちらか一方の生殖細胞のみをつくる。間性は体のすべての部分の細胞が同一の遺伝子型をもち、最初どちらか一方の性として発生した個体が、途中でホルモンなどの異常により、他の性に転換し、その途中の両方の性の中間でとどまることによって生じると考えられる。正常に雄と雌の両方の生殖器官や生殖細胞ができる雌雄同体hermaphroditeや、発生の途中で染色体の一部の欠失や組換え、突然変異などによって、体の一部が雄型で、他の一部が雌型になる雌雄モザイクsex mosaic, gynandromorphとも区別される。間性は雌雄性の程度によって、雌性間性、雄性間性、真正間性などに分けられる。

　間性はR・B・ゴルトシュミットが、森林の害虫であるマイマイガLymantria disparの日本種とヨーロッパ種の間でかけ合わせを行うと、子孫の間に一定の法則に基づいた種々の程度の間性をみいだした（1915）のが最初で、日本種とヨーロッパ種とで雌雄を決める因子の強さに差があるために、異なる種族のかけ合わせでは間性ができると説明された。ショウジョウバエでも間性ができ、この場合は性染色体の数と常染色体の組数との比によって性が決定され、その比が2：2のときは正常雌、1：2のときは正常雄、2：3のときは間性となる。また、ヤギやブタなどの家畜にもある。

　ヒトの間性は半陰陽とよばれ、性染色体異常（XXY。不活性化したX染色体の増加）により精巣が発達しないクラインフェルター症候群や同じく性染色体異常（XO。性染色体がX染色体1本のみ）で卵巣が発達しないターナー症候群、精巣と卵巣が存在する真性半陰陽など性腺(せいせん)の分化異常によるものが多い。

［黒田行昭］

『田中義麿著『基礎遺伝学』（1951・裳華房）』▽『駒井卓著『人類の遺伝学』（1966・培風館）』▽『市川衛著『基礎発生学概論』新版（1968・裳華房）』▽『R・H・タマリン著、木村資生監訳、福田一郎他訳『遺伝学』上（1988・培風館）』

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