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Sex-Linked Inheritance in Human Beings - Colour blindness

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Estimated time: 10 minutes

CISCE: Class 12

Definition: Colour Blindness

Colour blindness is an X-linked recessive hereditary disorder in which a person is unable to distinguish between red and green colours due to the presence of a defective gene on the X-chromosome.

CISCE: Class 12

Colour Blindness

The disorder is related to the cone cells of the retina.
A recessive allele responsible for colour blindness is represented as Xᶜ.
Since the gene is located on the X chromosome, males are affected more easily because they have only one X chromosome.
Females usually become carriers unless both X chromosomes carry the defective allele.

Genotypes

Individual	Genotype	Condition
Normal female	XᴺXᴺ	Normal vision
Carrier female	XᴺXᶜ	Carrier
Colour-blind female	XᶜXᶜ	Colour-blind
Normal male	XᴺY	Normal vision
Colour-blind male	XᶜY	Colour-blind

CISCE: Class 12

Inheritance Pattern

Colour blindness shows sex-linked inheritance and also follows criss-cross inheritance.

Criss-cross inheritance:

In this pattern, the trait passes:

from father to daughter,
then from daughter to grandson.

This means the trait often moves from one sex to the opposite sex in the next generation.

CISCE: Class 12

Important Crosses

1. Colour-blind male × Normal female

Result:

All daughters become carriers.
All sons remain normal.

2. Carrier female × Normal male

Result:

50% daughters may be normal.
50% daughters may be carriers.
50% sons may be normal.
50% sons may be colour-blind.

Maharashtra State Board: Class 12
CISCE: Class 12

Key Points: Colour Blindness

Colour blindness is an X-linked recessive disorder.
It mainly affects the ability to distinguish red and green colours.
It is caused by a defect associated with the retina / cone cells.
Males are more commonly affected than females.
Females may act as carriers.
It follows criss-cross inheritance.
A colour-blind father can pass the gene to his daughter, who may pass it to her son.