Both parents may be normal in phenotype, but htey may be carriers of the abnormality. Let us take an example of colour blindness. The red/green colour blindness which is sex linked and recessive i.e. the allele is carried on the non homologous portion of the X sex chromosome.

Since both parents are normal, it means that the mother is heterozygous i.e a carrier XCXc and the father XCY.
The son inherits his X chromosome from his mother and in this case received the X chromosome with the recessive allele.
Parental Phenotype Mother Father

Parental genotype of mother - XCXc

Parental genotype of father - XCY 

Parental gametes XC Xc XC Y

Offspring genotype XC XC XCY XCXc XcY

This shows a probability of 1 in 4 (25 %) of the child being colour blind.