Updated by: Arabinda Naik
Because of the way sex-linked inheritance works in humans. In humans, males have one X chromosome and one Y chromosome (XY), while females have two X chromosomes (XX). The key reason for this inability to pass on X-linked genes to sons is based on the pattern of inheritance associated with the X and Y chromosomes.
In the case of X-linked genes, they are located on the X chromosome. When a man (XY) contributes genetic material to his offspring, he passes on his Y chromosome to his sons and his X chromosome to his daughters. However, since his son receives his Y chromosome, the X-linked genes from the father are not passed on to the son.
The man has one X chromosome and one Y chromosome (XY). When he produces sperm, he contributes either an X or a Y chromosome to his offspring. If he contributes an X chromosome, the offspring will be female (XX). If he contributes a Y chromosome, the offspring will be male (XY). So, the X-linked genes that a man has on his X chromosome are transmitted to his daughters (who receive one X chromosome from him) but not to his sons (who inherit his Y chromosome).This is why X-linked traits or conditions are typically passed from mothers to their sons, as the mother contributes one of her X chromosomes to her sons.