Question

What is the type of genetic inheritance of color blindness? Is color blindness more frequent in men or in women? What is the physiological explanation for color blindness?

Answer 1

Most of us share a common color vision sensory experience. Some people, however, have a color vision deficiency, which means their perception of colors is different from what most of us see. The most severe forms of these deficiencies are referred to as color blindness. People with color blindness aren’t aware of differences among colors that are obvious to the rest of us. People who don’t have the more severe types of color blindness may not even be aware of their condition unless they’re tested in a clinic or laboratory.

Inherited color blindness is caused by abnormal photopigments. These color-detecting molecules are located in cone-shaped cells within the retina, called cone cells. In humans, several genes are needed for the body to make photopigments, and defects in these genes can lead to color blindness.

There are three main kinds of color blindness, based on photopigment defects in the three different kinds of cones that respond to blue, green, and red light. Red-green color blindness is the most common, followed by blue-yellow color blindness. A complete absence of color vision —total color blindness – is rare.

Sometimes color blindness can be caused by physical or chemical damage to the eye, the optic nerve, or parts of the brain that process color information. Color vision can also decline with age, most often because of cataract - a clouding and yellowing of the eye’s lens.

Men are much more likely to be colorblind than women because the genes responsible for the most common, inherited color blindness are on the X chromosome. Males only have one X chromosome, while females have two X chromosomes. In females, a functional gene on only one of the X chromosomes is enough to compensate for the loss on the other. This kind of inheritance pattern is called X-linked, and primarily affects males. Inherited color blindness can be present at birth, begin in childhood, or not appear until the adult years.

Genes are bundled together on structures called chromosomes. One copy of each chromosome is passed by a parent at conception through egg and sperm cells. The X and Y chromosomes, known as sex chromosomes, determine whether a person is born female (XX) or male (XY) and also carry other traits not related to gender.

In X-linked inheritance, the mother carries the mutated gene on one of her X chromosomes and will pass on the mutated gene to 50 percent of her children. Because females have two X chromosomes, the effect of a mutation on one X chromosome is offset by the normal gene on the other X chromosome. In this case the mother will not have the disease, but she can pass on the mutated gene and so is called a carrier. If a mother is a carrier of an X-linked disease (and the father is not affected), there is a:

• 1 in 2 chance that a son will have the disease,
• 1 in 2 chance that a daughter will be a carrier of the disease,
• No chance that a daughter will have the disease.

In autosomal recessive inheritance, it takes two copies of the mutant gene to give rise to the disease. An individual who has one copy of a recessive gene mutation is known as a carrier. When two carriers have a child, there is a:

• 1 in 4 chance of having a child with the disease,
• 1 in 2 chance of having a child who is a carrier,
• 1 in 4 chance of having a child who neither has the disease nor is a carrier.

In autosomal dominant inheritance, it takes just one copy of the mutant gene to bring about the disease. When an affected parent with one dominant gene mutation has a child, there is a 1 in 2 chance that a child will inherit the disease.

Vision begins when light enters the eye and the cornea and lens focus it onto the retina, a thin layer of tissue at the back of the eye that contains millions of light-sensitive cells called photoreceptors. Some photoreceptors are shaped like rods and some are shaped like cones. In each eye there are many more rods than cones – approximately 120 million rods compared to only 6 million cones. Rods and cones both contain photopigment molecules that undergo a chemical change when they absorb light. This chemical change acts like an on-switch, triggering electrical signals that are then passed from the retina to the visual parts of the brain.

Rods and cones are different in how they respond to light. Rods are more responsive to dim light, which makes them useful for night vision. Cones are more responsive to bright light, such as in the daytime when light is plentiful.

Another important difference is that all rods contain only one photopigment, while cones contain one of three different photopigments. This makes cones sensitive to long (red), medium (green), or short (blue) wavelengths of light. The presence of three types of photopigments, each sensitive to a different part of the visual spectrum, is what gives us our rich color vision. Most of us have a full set of the three different cone photopigments and so we share a very similar color vision experience, but because the human eye and brain together translate light into color, therefore, each of us sees colors differently.