細胞分裂が起こるときに、染色体が失われてしまったり、うまく分離しなかったりして、正常な46本すべてが揃わないことがあります。これが 減数分裂 で起こってしまうと、染色体を1本欠いた配偶子ができたり、1本余分に染色体を持つ娘細胞ができてしまいます。このような場合、目に見える異常が現れ、死亡するに至ることもあります。
染色体が1本多くなる(トリソミー)
ダウン症候群は先天性の発達・成長の障害で、心疾患を伴うことが多い病気です。ダウン症は一昔前まで蒙古症(mongolism)とも呼ばれていましたが、この病気はヒトの21番染色体のが1本多くなることで起こります。このように、ヒトのような染色体を2本ずつもつ生物の染色体が3本になることをトリソミー(三染色体性)といいます。
染色体は同じ大きさのものが2本ずつある
染色体は大きいものから順番に番号がつけられており、トリソミーが起こってしまう染色体はどれも小さいものです。ダウン症を起こす第21番染色体の他にも、第13番や第18番の染色体が1本余分にあることでトリソミーが起こることが知られていますが、これらよりも小さな染色体でトリソミーが起こって生じる病気が知られていないのは、他の染色体が重複してしまうと、胚の時期に死亡してしまうからです。
ヒトの性別を決める染色体(性染色体)がトリソミーになったらどうなるのでしょうか。染色体は通常、女性がXX、男性がYYという染色体の構成をしていますが、たとえば、これがXXYとなることがあります。これはクラインフェルター症候群と呼ばれます。
クラインフェルター症候群のヒトは男性に似ます。しかし、完全には正常な男性の表現型にはならず、不妊になってしまいます。XXYになったときに、それが雄になるか雌になるかは種によって異なっており、たとえばショウジョウバエでは雌になります。
XXYYやXXXYなどの染色体になることもあり、そのヒトは男性に似ることになります。つまり、ヒトではY染色体が性別を決定しているのです。どのような原因で、性別が雄になるか雌になるかは生き物によって異なります。
では、XXXの表現型になったらどうなるのでしょうか。XXXという染色体を持つことは、文字通りトリプルエックス症候群と呼ばれます。そして、トリプルエックス「症候群」といわれていますが、そのヒトは正常で、かつXXのときと同じで女性です。
XXXの女性が正常なことは、Y染色体が生命の維持に必要な機能をもっていないことを意味します。普通の表現型であるXXの女性はY染色体がなくても生きています。なので、Y染色体はヒトを男性にする機能を持っているのですが、性の発達にとって必須ではない(なかったら女性になるだけ)ということがいえます。
つまり、ヒトは元来、女性で、Y染色体が加わることによって男性になるといえます。聖書では、最初の人間はアダムで、彼の背骨からイブという女性が作られたことになっていますが、実際は逆で、科学的には、女性の「体」から男性が生じるというのが正しいといえます。(だからといって、実際にイブの骨からアダムが作られたわけではありませんが…)
では、XYYではどのような生物になるでしょうか？
XYYではY染色体があるので男性になります。基本的に正常な男性ですが、とても背が高くなることが多いようです。また、XXY型の男性の大多数は正常なのですが、普通のXY型の男性に比べて服役する人の割合が高いと言われています。染色体が1本多いから犯罪を起こしてしまうのか、染色体が多いことで社会生活を送る上でなんらかの障害があり、それが犯罪へと導いてしまうのか。このことについては解明されていません。
トリソミーの危険性は母親の高齢化によって著しく増加します。母親が20歳の場合、ダウン症の子供が生まれる可能性は1000人に0.4人といわれているのに対し、45歳の場合は1000人に17人と報告されています。
このような染色体の不分離は父親側でも起こっていると思われますが、精子は大量に作られるために、受精することができる精子は正常な場合が多い可能性が高くなるなどという生殖上の理由にもより、父親の年齢とトリソミーが起こる頻度の関係についての詳細はわかっていません。
染色体が1本少なくなる(モノソミー)
モノソミーとは染色体が2本必要なところ、1本しかないことを指します。たとえば、性染色体がX染色体が1本しかないターナー症候群があげられます。
ターナー症候群の染色体型はXO型と呼ばれます。Oは「ない」ことを意味しています。Y染色体がないので、ターナー症候群では女性になり、性的に未発達となってしまいます。
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