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 determined by the pigment(色素) which is color particles in the back part of the iris(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 activated to produce the melanin. And that’s determined by their genes. 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 blues 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.