Whether a baby develops as a boy or girl depends on a single gene on the Y chromosome. In humans and other mammals, females have two X chromosomes, but males have a single X, and a Y that bears the testis-determining gene (SRY) that induces testis differentiation and switches on hormones that masculinize the embryo. The X and Y are both weird chromosomes; far from having been “intelligently designed”, they are both the products of unique evolutionary forces. To understand these, we compare the chromosomes, genes and DNA in distantly related mammals and even birds and reptiles with completely different sex determining systems. The genomes of Australia’s unique fauna are a goldmine of new information.
The human X is a middle-sized, ordinary-looking chromosome, but is full of genes involved in reproduction and intelligence (often both). Since reproduction and intelligence are vital in the evolution of our species, the X has been called “the engine of evolution”. The tiny Y is a genetic wasteland – full of genetic junk and bearing only 45 genes, most active only in testis. Kangaroo sex chromosomes reveal the original mammal sex chromosomes, while the bizarre platypus sex chromosomes (more related to those of birds) tell us that our sex chromosomes are relatively young. I predict that the Y will disappear in just 5 million years; if humans don’t become extinct, new sex determining genes and chromosomes will evolve, maybe leading to the evolution of new hominid species.
一個胎兒要發育成雄性或雌性，是取決於Y染色體上的一個基因。在人類和其他哺乳類動物，雌性有兩套X染色體；而雄性只有一套X染色體和另一套Y染色體。這Y染色體上帶有「決定睪丸基因(SRY)」，它可啟動睪丸分化，造成胎兒雄性化。X和Y是兩組詭異的染色體，兩者都不算是被有智慧的設計，而卻是很獨特的演化力量的產物。為了要瞭解這些特性，我們我們比較了親屬關係很遠的哺乳類的染色體、基因和DNA，甚至也拿性別決定系統與哺乳類不同的鳥類和爬蟲類作對比。澳洲獨特的動物相的基因組這個寶藏，為我們提供了最新的訊息。
人類的X染色體是個中等大小，看來很平凡的染色體，但卻充滿了決定生殖和智力的基因。由於生殖和智力對我們人類的演化是不可或缺的，因而X染色體被稱之為「演化的引擎」。而小小的Y染色體，則被認為是「基因的廢棄場」，有一堆沒用的基因廢棄物。全部只有45個基因，也只有在睪丸裡才有功能。袋鼠的性染色體告訴了我們哺乳類動物性染色體的起源。而與鳥類很接近的鴨嘴獸的性染色體，則顯示出我們的性染色體其實是很年輕的。我預測Y染色體在五百萬年內會消失掉。若是人類沒滅種的話，那麼應該會有新的決定性別的基因和染色體演化出來，而可能演化出新的人種。 |
