Introduction
On the genes that are located on the X and Y chromosomes (sex chromosomes), the inheritance patterns are not the same than the ones applicable for the genes that are located on the 1-22 chromosomes. The main reason is that females are carrying two chromosomes X, and males are carrying a single X and Y chromosome each. As males are having a single chromosome X, any gene mutations of this chromosome can have as result X linked diseases. The reason is that for a female that is carrying two copies of genes linked to the X chromosome, the probability of inheriting the disease is quite high (compared to males), as this can be made in either recessive or dominant manner. For X linked diseases inherited through the dominant manner, any mutation of a copy of the X chromosome will result in a disease. Families with disorders related to the X linked dominant have registered both females and males affected in each generation, but with a predominance among males.
Inheritance Pattern
For the X linked diseases the inheritance pattern is a little complicated, mainly due to the fact that in all situations the male will pass the chromosome X to the daughter but not to their sons, while a female will pass the chromosome X to both sons and daughters. Diseases linked to the X chromosome can either be dominant or recessive. When the disease is coming with recessive manner, the most encountered symptoms are the red-green color blindness, various muscular dystrophy forms and Hemophilia. These diseases are more predominant in males than females, due to the fact that two copies of mutant gene are required for a disease to occur in a female, while for males one copy can be enough.
X-linked Dominant
Females and males can both be affected by X-linked dominant disorders, but males are having a higher probability. Chances of passing an X-linked dominant disease are different at males and females. Therefore none the sons of a male having an X-linked dominant disorder will inherit the disorder, but all the daughters will be affected by this condition. The fetus of a woman with an X-linked dominant disease is having 50% chances to be affected. Most common X linked diseases inherited in a dominant manner are: Aicardi syndrome, Incontinentia Pigmenti, the Rett Syndrome, and in rare cases the Klinefelter syndrome.
X-linked Recessive
The conditions related to X-linked recessive manner are more frequently registered at males than females. Again, chances of passing the disorder are varying from male to female. None the sons of a male having an X-linked recessive disorder will inherit the disorder, and the daughters will carry only a single copy of the gene affected by the mutation. A woman with an X-linked recessive condition is having 50% chances of having affected sons, and 50% of having daughters carrying one copy of the mutated gene. The X linked diseases produced in recessive manner are the Muscular Dystrophy Duchene, Hemophilia A, and the Lesch-Nyhan syndrome. Less serious disorders are the red-green color blindness and male pattern baldness.
