The dreaded disease known as hemophilia is the result of an inherited gene, and almost always strikes boys rather than girls (though mothers may pass the gene to their sons). Since the blood lacks an ingredient that causes it to clot or coagulate when a blood vessel breaks, even a minor wound can cause a hemophiliac to bleed to death if not treated. Bleeding can be particularly dangerous when it's entirely internal, with no visible wound, since the person may not be aware it's happening. Queen Victoria transmitted the hemophilia gene to royal families all across Europe; the hemophilia of a young Russian prince played a part in the downfall of the Russian czars. Today, hemophiliacs take drugs that stop the bleeding by speeding coagulation, and hemophiliac life expectancies in developed countries are almost as long as the average.