X-linked recessive is one of the possible ways that genetic traits can be inherited. This pattern is similar to autosomal recessive genetic inheritance in that one copy of the normal allele is enough to hide the phenotype of the recessive trait. However, since in this case the gene is located in the X chromosome (men only have one X chromosome, while women have two X chromosomes), the gender of the affected parent play a significant role in the inheritance pattern among sons and daughters.
If the altered gene version is responsible for a disease, a man with one altered allele in his only one X chromosome or a woman with two recessive altered alleles will be affected and develop the altered phenotype associated to this disorder (orange color in the figures) instead of the normal one (grey color in the figures). An heterozygous woman with one altered and one normal copies of the gene would not be affected by the disease, but she is a carrier and could pass the altered gene on to her children.
If the mother is affected by a X-linked recessive disease, she will transmit the disorder to all her sons but to none of her daughters (although they will inherit a copy of the altered allele and be carriers of the disease) as can be seen in figure 1:
Figure 1. X-linked recessive inheritance pattern when the mother is affected.
However, if the father is the one affected by the X-linked recessive disorder, none of the children will be affected, although the daughters will be carriers (figure 2). When a X-linked recessive carrier woman has children, daughters have a 50% chance of being carriers and sons a 50% chance of being affected by the disease (figure 3).
Figure 2. X-linked recessive inheritance pattern when the father is affected.
Figure 3. X-linked recessive inheritance pattern when the mother is a carrier.
Examples of genetic conditions that are inherited following an X-linked recessive pattern: