Color Blindness Genetics

Genes are the first and the most important gifts we receive from our parents. One set of chromosomes each from parents make up the genetic information of the progeny and is inherited by all organisms. The basic genetic modes of inheritance is X-linked inheritance.

X- Linked Genetic Modes of Inheritance
The X-linked genetic modes of inheritance are the traits or genetic disorders seen in the progeny due to the presence of a gene on the X chromosome. There are two X-chromosomes in females and males have one X and one Y chromosome. The genes present on the X are recessive or dominant and the way they are expressed in either of the sexes is different. This is because there are two copies of a gene on the X-chromosome in females and males have only one copy of an X-linked gene, which leads to expression of the trait or disorder. Many women are carriers of the genes but do not express the trait, but unknowingly pass it on to their sons, who may exhibit the trait or disorder. Thus, many women who are carriers of color blindness genes, inadvertently pass on the color blindness inheritance to their sons.

Types of Color Blindness
Color blindness is a condition that affects the person's perception of colors. These discrepancies lead to changes in color vision, that range from mild difficulty in recognizing shades, to total inability of detecting colors. There are three main types of color blindness:

• Red-Green color vision defect
• Blue-Yellow color vision defect
• Total absence of color vision

The most common color blindness inheritance reported is the red-green color vision defect. These people see colors differently and many a times cannot differentiate between different color hues. Blue-green color blindness inheritance is a rare type, that causes difficulty in differentiating between blue and green. The people with the above two conditions, just lack the ability to perceive color, and have a normal visual acuity. Total absence of color vision is called achromatopsia. This is a very rare and unusual deficiency, where people cannot perceive or visualize any colors. These people can only see shades of gray, black and white. These people may also suffer from eye problems such as photophobia, minor involuntary eye movements called as nystagmus and reduced visual acuity.

Genetics of Color Blindness
The traits of color blindness are determined by genes showing sex-linked genetic modes of inheritance. The most common color blindness as discussed above is the red and green color blindness. In color blindness genetics, the gene that determines color blindness is a recessive trait, whereas normal color vision is the dominant trait. Let us understand the color blindness genetics in a very simple way.

As we learn more about color blindness genetics, I will describe the gene for normal vision as 'B' and the gene for color blindness with 'b'. I will take different case studies to explain the genetics of color blindness.

Genetics of Color Blindness - Case # 1
Let us take into consideration a healthy couple Paul and Paula. Paula is color blind and Paul has normal vision. Paula is a carrier of two X chromosomes that have color blind 'b' genes. Paul has a X chromosome with 'B' color gene and Y chromosome that does not carry the color vision gene. When Paula conceives, the female fetus will carry one X chromosome with 'B' gene and the other 'b' gene. Paula and Paul have a baby girl, let us call her Mary, with normal color vision. Mary will be now a carrier of the color blindness gene 'b'.

Genetics of Color Blindness - Case # 2
When Mary gets married to John, with normal color vision 'B', they will have daughters with normal color vision, but they will be the carriers of the gene like their mother. Half of John and Mary's sons will have the color blindness inheritance, as they will have a set of X chromosome with the 'b' gene from Mary and will exhibit colorblindness. The other half of their sons will inherit the 'B' gene and have normal vision.

Genetics of Color Blindness - Case # 3
When a color blind Ray 'b' is married to Anne with 'BB' as color vision genes, they will have all off springs with normal vision. Their sons will be free from the recessive gene 'b', all their daughters will be carriers of the color blindness gene 'Bb'. These daughters will pass off this trait to half of their children in future.

I will explain further the color blindness genetics with the help of 'punnett square'. This is a device, that helps predict the probability of off springs, that will exhibit certain genetic traits.

Case # 1
Normal color vision man 'XY-B' and a carrier women 'XX- Cc' may have offspring with the following genetic traits:

	XB	Y
XB	XB XB	XB Y
Xb	XC Xb	Xb Y

This table shows that the daughters will have normal vision and one of them will be a carrier of color blindness. One son will have normal color vision and the other will be color blind.

Case # 2
A color blind man with Xb Y genes and woman with normal color vision and who is a carrier of the color blindness gene XB Xb will have children with genetic make up as follows:

	Xb	Y
XB	XB Xb	XB Y
Xb	Xb Xb	Xb Y

This square shows that one of their daughter will be color blind and the other a carrier of color blindness gene. One of the son will have normal vision and the other will be color blind.

Case # 3
Let us consider a remote possibility that a color blind man and color blind women are to have children, then they will have a genetic make up comprising of:

	Xb	Y
Xb	Xb Xb	Xb Y
Xb	Xb Xb	Xb Y

All their daughters and sons will be color blind. This is a highly remote occurrence where the entire family is color blind!

Color Blindness Facts

• Color blind people are not actually blind, but are color deficient. The more appropriate term to be used for color blindness is, color vision deficiency.
• 99% of the people suffer from red-green color vision deficiency. Of these, 75% have trouble with perception of green color and 24 % have trouble with perception of red.
• Some types of color blindness are not genetic. They develop after birth and the most common types of this color blindness are caused by a drug Plaquenil, used to treat rheumatoid arthritis.
• Color blindness inheritance is not passed on from the color blind father to his son.
• The first known scientific paper on color blindness was written by John Dalton, who himself was color blind.
• Color blindness or color vision deficiency is also called as 'Daltonism', after John Dalton.
• The approximate percentage of color blindness in women is 0.5% only.
• There are different chromosomes that are involved in occurrence of different types color blindness.
• Color blindness was the first genetic trait that was identified and linked to a specific chromosome.
• Pseudoisochromatic plates, arrangement test, and the anomaloscope are the most common color blindness.
• There is no cure for color blindness

These were a few color blindness facts and an over view of color blindness genetics. This sex-linked inheritance causes some amount of difficulty in recognizing and differentiating colors. Genetics of color blindness is very difficult to understand. But try using punnett square, with all the possible genetic combinations you can think of and find an answer to your questions on color blindness genetics.