A faulty recessive allele on the X chromosome causes colour blindness.

Colour blindness occurs when you are unable to see colours in a normal way based on red-green-blue. It often happens when someone cannot distinguish between certain colours. This usually happens between greens and reds, and occasionally blues.

Because colour blindness disorder is sex-linked, both the chromosome and the allele must included in the genetic diagram showing the possible offspring genotypes and phenotypes.

Women need two copies of the recessive allele to be colour blind, whereas men need only one copy.

A woman with only one copy of the recessive allele is a carrier - in other words, she is not colour blind herself but can pass on the recessive allele for colour blindness to her offspring.

Example 1 of inheriting colour blindness

In the diagram below crossing an unaffected female carrier of the faulty allele with an unaffected non-carrier male, superscripts N represents the normal allele and n the recessive faulty allele.

In this particular cross the ratio of unaffected to colour blind is 3 : 1 (25% chance of the offspring being colour blind).

The ratio using a

unaffected non-carrier (male + female) : unaffected female carrier : colour blind male is 2 : 1 : 1

Genetic Punnett square table for crossing an unaffected female carrier of the faulty recessive gene for colour blindness and an unaffected male (non-carrier)
Parent genotypes cross: XNXn  x  XNY
Gametes: XN, Xn , XN and Y
Genotypes of parents - gametes - alleles	XN	Xn
XN	XNXN	XNXn
Y	XNY	XnY

 

Example 2 of inheriting colour blindness

A woman with one copy of the recessive allele is a carrier of colour blindness, but not colour blind herself.

She is still a carrier though, whereas a man only needs one copy of the faulty allele to be colour blind.

However, a woman needs two copies of the recessive allele to be colour blind herself, so colour blindness is less common (less chance) in females.

Genetic Punnett table for crossing an unaffected female carrier of the faulty recessive gene for colour blindness and an affected male (carrier)
Parent genotypes cross: XNXn  x  XnY
Gametes: XN, Xn , XN and Y
Genotypes of parents - gametes - alleles	XN	Xn
Xn	XNXn	XnXn
Y	XNY	XnY

Here 3 out of 4 chance (75%) of the offspring children are likely to be carriers of the recessive allele.

There is a 2 in 4 (50%) chance the offspring will be affected by colour blindness, but of these, half are likely to be male and the other half female.

i.e. there for there is a 1 in 4 chance (25%) that the offspring will be male and affected OR the offspring will be female and affected.