Single-gene diseases
Investigation of the causes of recurrent miscarriage
Sex-linked genetic diseases (X-linked)
Women have two X chromosomes (XX), while men have one X chromosome and one Y chromosome (XY).
During fertilization, the female gamete (egg) always contains one X chromosome, while the male gamete (sperm) can contain either an X or a Y chromosome. If the sperm carrying an X chromosome fertilizes the egg, the resulting embryo will be female. If the sperm carrying a Y chromosome fertilizes the egg, the resulting embryo will be male.
If the disease-causing gene is located on the X chromosome, it can be passed from the mother to all of her children, but from the father only to his daughters. This sex-linked disease-causing gene can be dominant or recessive.
We know that a dominant gene always causes the disease, regardless of whether the other chromosome’s gene is healthy or diseased, but a recessive gene only causes the disease when there is no healthy gene in the cell.
Therefore, if the disease gene is on the X chromosome, it will always manifest in males, but in females, it will only manifest when both X chromosomes carry the disease gene. More specifically, in recessive X-linked diseases, both X chromosomes in females must be defective for the disease to occur.
If one X chromosome is defective, the disease will not occur, but it can be passed on to offspring. In males, a defect in the only X chromosome causes the disease.
Examples of X-linked inherited diseases include fragile X syndrome, hemophilia, Duchenne muscular dystrophy, color blindness, vitamin D-resistant rickets, and Rett syndrome.