ここでは１つの遺伝子が他の座にある因子の作用を隠す現象「上位(epistasis)」について記述しています。
遺伝子の上位と下位
複数の遺伝子が同一の形質に働くとき、１つの遺伝子が他の遺伝子の作用を抑えることがあります。単純な対立遺伝子の優性と劣性に似ていますが、１つの遺伝子が異なる座にある因子に作用をする場合もあります。これが「上位」「下位」と呼ばれる現象です。
ハツカネズミの毛の色の例で考えてみましょう。C遺伝子は毛の色の有無を決める遺伝子で、ccの遺伝子型では個体は白子になります。BBやBbでは黒色になり、bbは褐色になります。しかし、Cの因子がなければ、BBもBbもbbも発色されません。
C- B- 黒色
C- bb 褐色
cc B- 白色
cc bb 白色
このような場合、c遺伝子はBやbの遺伝子対に対して上位(epistasis)であるといいます。そして、Bやbはｃ遺伝子に対して下位(hypostatic)であるといいます。ccという遺伝型によってBとｂの区別が抑えられることになるからです。
また、上位・下位のある場合の、交配における表現型の分離比は通常とは少し異なります。たとえば、CcBbの遺伝子型を持つ黒色の個体同士を交配すると以下の表のようになります。
黒色：褐色：白色＝９：３：４となります。通常の９：３：３：１と同じですが、表現型において右下の４枠の遺伝子型はすべて同一の白色個体となります。
また変異遺伝子(modifier)として知られている遺伝子もあり、これは他の遺伝子の表現を変える働きを行いますが、それ以外には効果がありません。たとえば、ホルスタインウシに白斑を生じさせる遺伝子がありますが、いくつかの変更遺伝子が白斑の大きさに影響を与えます。変更遺伝子の個数や存在する場所によって、固体のほとんどが白色になったり、ほとんど黒色になったりましす。
このような遺伝子の相互作用は、哺乳類や鳥類ほとんど全般で観察されます。このような遺伝子の上位関係や優性の関係は、遺伝子が生産する酵素について考えることで説明ができます。
酵素による遺伝子の制御
酵素は化学反応を触媒するために、わずかな量だけあればよいものです。なので、１つの遺伝子が原物質を生産物にカエルのに十分な酵素を生産することができます。なので、CCの遺伝子型を持つ個体が、Ccの遺伝子型を持つ個体より二倍の酵素を生産することができたとしても、どちらの個体も同じ量の色素を持つことができます。
上記のハツカネズミの毛の色に関して、Cという遺伝子が色の有無を生じさせていました。C遺伝子はアミノ酸のチロシンをメラニンに変える酵素を生産します。c遺伝子は酵素を作らないので、ccの遺伝子型を持つ個体は白色になってしまいますが、CCやCcの遺伝子型を持つ個体はどちらも有色の個体を作るに十分な酵素を生産します。
実際には、Ccの遺伝子型を持つ個体には少数の白い毛が生じることがあります。なので、遺伝子が生産する酵素の量を直接測定すれば、このふたつの表現型を区別することができます。
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