Colour Blindness

Red-green colour blindness is the inability to distinguish red and green colours by some people. This trait is linked to the X chromosome.

The gene that determines normal colour vision is dominant over that for colour blindness.

A marriage between a colour-blind man and a woman homozygous for normal colour vision results in their daughters being carriers but with normal colour vision.

The daughters are described as carriers because they are heterozygous, and colour blindness is suppressed by the dominant gene for normal colour vision.

All the sons of the two parents, however, are normal.

Let N represent the gene for normal colour vision.

Let n represent the gene for colour-blindness.

Since the gene is linked to the X chromosome, its alleles are represented as X^N and Xⁿ.

Genetic cross

If a carrier daughter from above parents married a normal man, some of their sons will suffer from colour-blindness while the daughters will be either carriers or homozygous for normal colour vision.

The above example shows that the gene for colour-blindness is passed from the mother to sons. This is because the only X chromosome the male offspring inherits is from the mother.

Consequently, there are more male sufferers in a population compared to females. Females only suffer when in a homozygous condition of the recessive gene.

If the X chromosome carries the gene for the trait, then this gene will be expressed since the allele on the Y chromosome is absent.

A pedigree for colour blindness trait

A pedigree is a record, in table form, showing the distribution of one or more traits in different generations of related individuals.