Most of us learned what we know about eye color from a chart in grade-school biology. You know two blue-eyed parents are going to have blue-eyed kids. But the story of how eye color is passed down is more complicated（复杂的) than we’re taught.

Humans get their eye color from melanin（黑色素）, which also determines skin and hair shades. Melanin is good at absorbing light, which is especially important for the iris（虹膜）, the function of which is to control how much brightness can enter the eyes. Once it passes through the irises, the majority of the visible light goes to the retina（视网膜）, where it’s translated into images by the brain. The little that isn’t absorbed by the iris is reflected back, producing what we see as eye color. That color depends on the kind and amount of melanin a person is born with.

Though we used to think eye color just came from a simple pattern of inheritance （遗传）, in recent years scientists have found that it’s also shaped by many genes acting together. What’s more, tiny changes, or mutations（突变）, in a gene can result in different shades in the iris.

The two genes currently thought to be most strongly linked with human eye color are OCA2 and HERC2. OCA2, the gene we used to think was the only player in eye color, controls the production of the P-protein and the organelles that make and transport melanin. Different mutations in the OCA2 gene increase or decrease the amount of protein that’s produced in the body, changing how much melanin is sent to the irises. The HERC2 gene, meanwhile, acts like a helicopter parent （直升机父母） for OCA2. Different mutations in this gene act as a switch that turns OCA2 on and off and determines how much P-protein it produces.

Those are just the two genes we know about in detail so far. Newer studies have linked as many as 16 genes to eye color, all of which pair with OCA2 and HERC2 to generate different iris colors and patterns. With all these genes and their structural changes, it’s hard to say for sure what a child’s eye color will be based on their parents’.

Crossing your legs is an extremely common habit; most people don’t even notice that they’re doing it when they sit down. While you may find it comfortable to sit with one knee crossed over the other, it might be causing health problems that you are not aware of.

A study published in Blood Pressure Monitoring stated that sitting with your legs crossed can increase your blood pressure. Actually, the blood in your legs has to work against gravity to be pumped back to your heart. Crossing one leg over the other increases resistance, making it even harder for the blood to circulate. This causes your body to increase your blood pressure to push the blood back to the heart.

Crossing your legs can also lead to neck and back pain. When you sit with your legs crossed your hips are in a twisted position, which can cause one of your pelvic bones to turn a bit. Since your pelvic bone supports your neck and spine, this can cause pressure on your lower and middle back and neck.

You also might notice that when you sit with your legs crossed for long periods of time your feet and legs have the feeling of being asleep. This is because when one leg sits on top of the other the veins and nerves in them are pressed, thus bringing about temporary paralysis in the legs, ankles, or feet. While the feeling of discomfort may only last a minute or two, repeatedly crossing your legs until they feel numb can cause permanent nerve damage.

So next time you sit down, try to get yourself in the habit of sitting with both of your feet planted flat, hip width apart, on the floor. Not only will it help your posture and stability, but it will also save your health in the long run.

Think of the body like a school. At the top of the school is the headmaster, known as the brain. This is the organ that controls the rest of the organs, just like the headmaster controls the school. After this comes the heart. It is like a teacher. This organ pumps the blood around the body so it can run well. 【小题1】 Next is the liver, which is the only one that can regrow itself. It helps to clean the blood. This is like the knowledge students get from teachers. Students always grow with newly found knowledge. The lungs are the organs of the body that use the oxygen we breathe. Think of the oxygen as the life we lead at school. The stomach is very useful as it stores the food we eat until we can digest it properly. 【小题2】 We store the information until we need to use it. So this organ is like a notebook. The kidneys are the organs that remove waste products from the blood and produce urine. 【小题3】 Now you can see how similar our school life is to our body and we can look after them both.