英语听力：自然百科 为何婴儿大多拥有蓝色眼睛？

 Hi, it’s Kasey-Dee Gardner for Why? Tell Me Why? This week we received a question about baby eye color. The question was “why are most babies born with blue eyes?”.

And I wasn’t sure that was a true statement, but it is in fact a true statement.

 

Actually we think most babies are, I don’ t have an exact number for you, and to understand why, you have to understand where the color of the eye comes from.

 

The color of the eye is determined¹ by the pigment²(色素) which is color particles in the back part of the iris³(虹膜), and pigment in the main tissue of the iris. And these develop actually separately. So when a baby is born, the pigment in the back part of the iris, of the healthy iris, is already formed, and the pigment in the main tissue of the iris isn’t. This pigment is called melanin(黑色素).

 

A baby would be born with pigment right here on this bottom layer. And they would all be born the same way. Everybody has the cells that make melanin.

 

In the people who have blue eyes, those cells are just not activated⁴ to produce the melanin. And that’s determined by their genes⁵. In babies who go on to have brown eyes by six months or a year, their genes have determined that their cells will produce melanin and that happens over that first year of life. In babies who go on to have blue eyes, their cells are not programmed by the genes to produce melanin. And their eyes will remain blue as they were at birth.

 

So how many cells are stacked in there, how hard the light, how much the light has to do to get through all those cells, will also impact and it will create shades, of greens and blues⁶ and browns and hazels, because the light is bouncing around a lot.

 

So this is actually across races? Uh this is a, this has been normal development cycle of the eye. So a baby who will have darkly pigment in skin, and darkly pigment at eyes, ah, I say the one, one year mark, can be born with blue eyes, simply because the melanin has not yet been generated in the tissues of the iris.

1 determined   

adj.坚定的；有决心的

参考例句：

• I have determined on going to Tibet after graduation.我已决定毕业后去西藏。
• He determined to view the rooms behind the office.他决定查看一下办公室后面的房间。

2 pigment   

n.天然色素，干粉颜料

参考例句：

• The Romans used natural pigments on their fabrics and walls.古罗马人在织物和墙壁上使用天然颜料。
• Who thought he might know what the skin pigment phenomenon meant.他自认为可能知道皮肤色素出现这种现象到底是怎么回事。

3 iris   

n.虹膜，彩虹

参考例句：

• The opening of the iris is called the pupil.虹膜的开口处叫做瞳孔。
• This incredible human eye,complete with retina and iris,can be found in the Maldives.又是在马尔代夫，有这样一只难以置信的眼睛，连视网膜和虹膜都刻画齐全了。

4 activated   

adj. 激活的 动词activate的过去式和过去分词

参考例句：

• The canister is filled with activated charcoal.蒸气回收罐中充满了活性炭。

5 genes   

n.基因( gene的名词复数 )

参考例句：

• You have good genes from your parents, so you should live a long time. 你从父母那儿获得优良的基因，所以能够活得很长。 来自《简明英汉词典》
• Differences will help to reveal the functions of the genes. 它们间的差异将会帮助我们揭开基因多种功能。 来自英汉非文学 - 生命科学 - 生物技术的世纪

6 blues   

n.抑郁，沮丧；布鲁斯音乐

参考例句：

• She was in the back of a smoky bar singing the blues.她在烟雾弥漫的酒吧深处唱着布鲁斯歌曲。
• He was in the blues on account of his failure in business.他因事业失败而意志消沉。