Colour Blind Resource Centre - How do you get colour blindness?

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How does one get colour blindness?

NB: This Sites' explanation of the inheritance of colour blindness provides a crash course in biology and is of greater use if one has basic knowledge of biology.

Colour blindness is a sex linked recessive condition, which all you biology buffs know means that it is a recessive trait condition which is transmitted in the 23 pair of chromosomes.

Chromosomes are spaghetti-like strands containing genetic information (Like a blue print of how the body forms) and are found in the nucleus of all cells. Normal humans have 23 pairs of chromosome in all cells except gametes (sex cells). Males in the 23rd pair of Chromosomes have one X chromosome and one Y chromosome (which contains no genetic information). Females in their 23rd pair have two X chromosomes. As I mentioned before, colour blindness is a recessive trait which is transmitted via X chromosomes in the 23rd pair of chromosomes.

A recessive trait requires two of the recessive genes for an organism to develop the recessive characteristic. For example the genotype (genetic makeup) BB and Bb would both show the dominant characteristic. The genotype bb would show the recessive trait.

Seeing females have two X chromosomes in the 23rd pair, they can be carriers (normal vision but carrying the gene for colour blindness) of the condition, normal or affected. For the female to be a carrier, she must have one affected X chromosome (A chromosome containing the gene for colourblindness) and one Normal chromosome (A chromosome containing the gene for normal vision). To be normal the female would have two normal X chromosomes in her 23rd pair and to be affected she must have two affected chromosomes. In other words the genotype (genetic makeup) of the 23rd pair of chromosomes (while X=normal and X^R = affected) would be:

For a female with normal vision, XX

For a female with normal vision but a carrier, X^RX

For a female with colour blindness, X^RX^R

Males only have one X chromosome in their 23rd pair which means they can either have normal vision or be affected, they cannot be carriers. the genotype of the 23rd pair of chromosomes (while X=normal and X^R = affected) would be:

For a male with normal vision, XY

For a male with colour blindness, X^RY

Because males have only one X chromosome in their 23rd pair, they have a higher chance of inheriting colour blindness, this is why more males in the population have colour blindness then females.

For example if a Female carrier mated with a male with normal vision:

XX^R x XY

Punnet Square

	X^R	X
X	X^RX	XX
Y	X^RY	XY

The theoretical Ratio of the offspring would be 50% females with normal vision (50% of those carriers), 25% Affected males and 25% males with normal vision.

As you can see females are less prone to inheriting colour blindness.

Now I hope you have a basic understanding of how one gets colour blindness.

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