Sex-Linked Inheritance
If a trait appears equally often in males and in females then the trait is autosomal. In this case, we would look at the amount in both females and males to determine the result. If males are more likely to have the trait, it is X-linked.
The hemophilia gene is located on the X chromosome. Females have two Xs and therefore, two Factor genes; if one of her genes is mutated she can depend on the function of the other gene and escape the condition. Males however, have only one X and one Factor gene; if he has a mutated factor gene, he has no other copy to fall back on. Since this disorder is recessive it makes the females carriers and the males have a higher chance of inheriting the disorder.
A boy inherits Hemophilia when he inherits an X chromosome with the mutated factor gene from his mother. He inherits a Y chromosome from his father, but the Y is normal, it is not affected by a mutation; it does not contain the Factor gene and therefore, Hemophilia is inherited from the mutated gene he got from his mother. However, there is another way in which the boy can inherit hemophilia, and this is when his mother does not carry a mutated factor gene and is thus normal this happens during egg production; if the gene mutates during egg production or early in the development of the embryo. There is a 20% chance of this happening.
The severity of the boy’s hemophilia depends on the specific mutation in his factor gene; if the mutation is small like a one letter change in the gene then the symptoms are mild or moderate. If the mutation is larger like one large sequence of the gene is changed, then the symptoms are severe.
They benefit by having a second factor gene that can function above the other that has been mutated.
Females that are carriers for Hemophilia have mild symptoms.
1) Biology. Seventh Edition, Neil A. Campbell and Jane B. Reece. Pearson Eduction Inc., publishing as Benjamin Cummings, 1301 Sansome St., San Franscisco, CA
2) Biology. Peter H. Raven, George B. Johnson, Susan Singer, Jonathan Losos. McGraw-Hill Science/Engineering/Math; 7 edition (January 8, 2004)
3) DNA Science: A First Course, Second Edition. David A. Micklos. Cold Springs Harbor Laboratory Press; 2 edition (January 8, 2003)