Its In the Genes

　　它在基因里

　　by Barbara Eaglesham

　　Sometimes an unfortunate event will have far-reaching positive consequences. When Austrian Johann Mendel’s father died, Johann could no longer afford to continue his studies in science at an advanced local school. Instead, in 1843 he entered a monastery that had a reputation as a center for scientific study. Here, he took the Christian name Gregor and began experimenting with crossing pea plants in the monastery garden.

　　Mendel pollinated plants himself (before the insects could do so randomly), over and over, until offspring exactly resembled parents. Then he crossed plants with different traits －AA (red flowers) and bb (white flowers), for example. To his surprise, the first-generation plants looked like only one of the parents (red flowers－taking one trait from each parent, you get the combination Ab, which, mysteriously, looked like AA). Crossing these with each other (Ab×Ab) produced a mixture: 75 percent that looked like AA (red flowers－25 percent AA, 50 percent Ab) and 25 percent that looked like the lost parent, having white flowers (bb). Scientists of his day expected the first generation (Ab) to look like a blend of the two traits, instead of resembling just one parent.

　　From this work, Mendel realized that traits had to be carried in something like a package－a gene. He also realized that each offspring received one unit of information (chromosome) from each parent. He concluded that just because a trait is passed down, it isn’t necessarily visible (it might not be expressed). His hard work and insight weren’t recognized in his lifetime, but he eventually came to be known as the “Father of Genetics.”

　　We’ve learned a lot since then. Chromosomes are found in the nucleus of cells, and there are 46 (23 pairs) in every one of ours. One chromosome of each pair comes from one of our parents, the other chromosome from the other parent. Deoxyribonucleic acid, or DNA, is found in all chromosomes and carries the code of life. DNA is a double helix, shaped like a twisted ladder, whose rungs are made up of pairs of nitrogen base-molecules. There are four bases, called adenine (A), thymine (T), guanine (G), and cytosine (C). They fit together like puzzle pieces: A with T and G with C. Just the way the arrangement of letters in language carries meaning, so does the order of the bases carry a code.

　　有时，不幸的事件会产生深远的正面结果。当奥地利人约翰·孟德尔的父亲去世后，他再也没有钱在当地一所高等学校继续进行科学研究了。于是，1843年，他进了一所修道院。这所修道院被人们认为是一个科学研究的中心。到了这里，他使用自己的教名格里戈尔,并在修道院的花园里开始进行豌豆杂交试验。

　　在昆虫给这些豌豆随意授粉之前，孟德尔对它们反复进行人工授粉，直到它们的后代同它们外表完全一样为止。然后，他用不同特征的豌豆进行杂交——例如，AA（红花豌豆），bb（白花豌豆）。令他感到惊奇的是，第一代的豌豆外表仅仅同上一代豌豆中的一种类似。杂交的后代是由红花豌豆和白花豌豆结合产生的（Ab），但它却奇怪地与红花豌豆（AA）非常相似。这种豌豆再相互杂交（Ab×Ab）后所产生的后代就不只是一种了：75%像红花（25%的AA，50%的Ab）；25%表现出第一代失去的特征：白花（bb）;当时的科学家预想第一代（Ab）表现出父母结合起来的特征，而不仅仅表现出父母一方的特征。

　　从这项研究中，孟德尔意识到植物的特征应该像“包裹”般传递：这就是基因。他还意识到每一个后代从父母双方各得到了一个染色体。他的结论是：特征即使传给后代，也并不一定会在后代中显现。在他有生之年，他的辛勤工作和远见卓识并未得到重视，但他后来渐渐地被称为“遗传学之父”。

　　从那时起，我们了解了很多东西。染色体存在于细胞核内。我们每人都有46个染色体（23对）。每一对染色体中的一个来自父亲，另一个来自母亲。DNA在所有染色体中都存在，并且它携带有生命密码。DNA是双螺旋状物，就像一架扭曲的阶梯。而梯子的横档是由一对对含氮碱基分子构成。共有四个碱基：腺嘌呤（A）、胸腺嘧啶（T）、鸟嘌呤（G）和胞嘧啶（C）。它们像积木一样拼在一起：A和T在一起，G和C在一起。就像语言中的字母排列会产生意义一样，这些碱基也带有遗传密码。