Answer: Women are carriers for the disease hemophilia because hemophilia is an X-linked disease. The recessive allele for the disease is found on the X chromosome.
The sons of carrier women get hemophilia because males have one X chromosome and one Y chromosome. The sons of a carrier woman may inherit the affected X chromosome from their mother, and a Y chromosome from their father. They have only one X chromosome and one it's affected, will result in hemophilia.
Explanation: Women are carriers because in most cases, only one of their two X chromosomes are affected. The two female X chromosome is rarely affected.
Daughters of carrier women will not get the disease because they need two affected X chromosomes to manifest the disease. So even if they inherit one affected X chromosome from their mother, the second X chromosome they will inherit from their father will be normal. Therefore, daughters of carrier women can only be carriers.
