Disease linked to the X of special relevance neurogenetics.

Recessive X-linked inheritance pattern:

• The frequency of sick individuals is much higher among men than among women.
• Men pass the diseased allele to all their daughters, but all daughters will be healthy.
• A heterozygous carrier will transmit the allele to 50% of her sons (who will be sick) as well as 50% of her daughters (who will be healthy carriers).
• There is no transmission of the disease from parents to children.

Dominant X-linked inheritance pattern:

• Both men and women transmit the disease, and it is expressed in both sexes.
• The disease is twice as common in women as in men.
• An affected man transmits the disease to all his daughters, but to none of his sons.
• An affected woman transmits the disease to half of her sons and daughters.
• Women usually have a milder form of the disease than men.

Hollandric or Y-linked inheritance.

• Only men express and transmit the genes linked to the y chromosome, and they pass them on to all their male children.

Pseudoautosomal inheritance or “partial sex linkage”.

• Women transmit the genes of the X/Y homologous segment in the same way to their children of both sexes, but men transmit them predominantly to children of the same sex. For example, this inheritance model occurs in X-linked ichthyosis due to steroid sulfatase deficiency.

Since X-linked diseases are more limited in number than autosomal diseases, it is a better strategy to know and learn about them. An extensive list can be consulted at OMIM.