Such parents are called carriers of the disorder. When both parents carry the same defective allele of a gene, each child they have has a chance of one in four of having the disorder. An example of an autosomal recessive disorder is cystic fibrosis. This type of inheritance refers to genes that are carried on the X chromosome, one of the sex chromosomes. As females have two X chromosomes, they have two alleles for each X-linked gene.
Males only have one X chromosome and hence have only one allele for each X-linked gene.
X-linked diseases or disorders are usually recessive diseases and occur more frequently in males than in females. This is because a male with one defective allele of a gene will show the defect. A female with one defective allele of a gene may have a normal allele of the gene on the second X chromosome. The presence of one normal allele may result in normal development in the female, and so the presence of the defective allele is masked. A normal female who has one normal and one defective allele of a gene is called a carrier of the defect.