Colourblindness: 

It is a hereditary eye disease. It was first described by Horner. It is of 2 types – 

1. Red-green colourblindness 

2.Blue colourblindness .

• The red-green colourblindness is most common & it is also called a proton defect. 
• Red-green colourblindness is caused due to the absence of red-green cones in the retina.
• The person is unable to identify red, green, yellow & orange colours. These colours appear to be green in the absence of red cones and they appear to be red in the absence of green cones.
• It is a sex-linked disease which is caused due to a recessive gene on the x-chromosome. The mother (female) acts as a carrier. 
• The father of a colourblind daughter is always colourblind. 
• In the F₁ generation of carrier mother & normal father, 50% generation will be normal, 25% will be carrier & 25% will be colourblind. 
• In F₁ generation of carrier mother and colourblind father, 50% of daughters will be colourblind & 50% will be carrier likely, 50% of boys will be normal & 50% will be colourblind.
• In the F₁ generation of normal mother & colourblind father, all males will be normal and all daughters will be a carrier.