If the gene locus is on the lower half of the X chromosome then a problem occurs because in mammals the male has only one X chromosome. The male’s Y chromosome lacks the normal bottom half of the X and so does not have the normal range of alleles present. This causes certain defects to occur more frequently in males than in females, for example haemophilia and red/green colour vision deficiency. It works like this:

A female with one normal and one recessive defective allele mates with a normal male, the” - “represents the male’s Y chromosome. We get the expected 3 dominant: 1 recessive ratio but the recessive is the male. Because the female would need to receive 2 defective alleles to develop the recessive phenotype it is much less likely to occur. ( To pick up a sex linked phenotype look for evidence of female to male transmission of the faulty allele, and note that the male can never pass the sex-linked recessive allele to a son, it always comes from the mother, since she gave the only X chromosome the son possesses.)