Occurs when the gene occurs on the sex chromosomes

Males cannot be carriers, if the faulty allele is on the X chromosome, as they will only have one copy of the gene.

Men tend to be much more affected, as women must have both faulty alleles, whereas men only need have one

- red/green colour blindness: recessive on X chromosome

- pattern baldness: dominant on Y chromosome

- haemophilia: recessive on X chromosome