Sex-linked Characteristics

When alleles that control a particular characteristic are found on the sex chromosomes, we describe the inheritance that results as ‘sex linked’
In almost all cases, there are only alleles on the X chromosome as the Y chromosome is much smaller
Because males only have one X chromosome, they are much more likely to show sex-linked recessive conditions (such as red-green colour blindness and haemophilia)
Females, having two copies of the X chromosome, are likely to inherit one dominant allele that masks the effect of the recessive allele
A female with one recessive allele masked in this way is known as a carrier; she doesn’t have the disease, but she has a 50% chance of passing it on to her offspring
If that offspring is a male, he will have the disease
The results of a cross between a normal male and a female who is a carrier for colourblindness is as follows:

Punnett square showing the inheritance of colourblindness, an X-linked condition

In the cross above, there is a 25% chance of producing a male who is colourblind, a 25% chance of producing a female carrier, a 25% chance of producing a normal female and a 25% chance of producing a normal male