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http://communityhigh.org/usr/graham/hemophilia/
Genetic Makeup
 

     Hemophilia is a genetic disease. This means that the disease is passed down through the generations by means of our genetic codes, or genes. Humans are made up of 46 chromosomes which have the battle plans for various functions, actions, and makeup of the body. These are 23 chromosome pairs. Chromosomes also determine whether or not we will be male or female. These two chromosomes labeled "x" and "y". A male will have one "x" and one "y" and females will have two "x"'s. Usually males are the only ones who get hemophilia. Hemophilia is found on the "x" chromosome at the very bottom and when it occurs people suffer from the inability to form firm clots. Which, means in short, that they bleed, and keep bleeding.
 
   The reason that more males get the disease is quite simple. Since the  disease occurs on the "x" chromosome, if a male's "x" chromosome has the disease, he doesn't have another "x" to block it out. But a female on the other hand has two "x" chromosomes so if one has the disease, she has another "x" to block it out. So, if a female has only one "x" diseased she is called a carrier, so she has the disease in her, but it doesn't show up. A male is always a hemophiliac.
 
    In reality there are two different kinds of hemophilia that act alike. Hemophilia A (factor VIII deficiency) and Hemophilia B (factor IX deficiency) This protein deficiency causes the blood to not be able to form a firm clot, and thus a hemophiliac who gets bruised or cut, keeps on bleeding. Not more, but longer. There are also other blood clotting disorders such as von Willebrand disease which can affects both sexes.
 

Generations
 
      One of the most scary and interesting things about genetic diseases is the fact that they are passed down through generations of people. They often don't show up for a couple of generations and then come back in full force. This puzzled many scientists long ago, but then tools were developed that can map and show how these diseases are passed down. To understand what is to come, certain things must be clear. Meiosis is THE crutial part of passing down genetic diseases. Meiosis is a process by which cells divide and mulitply down until the male's cell becomes sperm cells, and the female's cell becomes an egg. Also, in the diagrams below, "xy" means male, and "xx" means female, and if there is a "h" by it, that chromosome carries the disease. Only a "x" chromosome will carry hemophilia.
GENERATIONS
     The first of the tools scientists use to track diseases is called a pedigree. A pedigree shows all the males and females of a family and then shows which person had what. Carriers are marked by half the shape being filled in, and people which have and show the disease are completely filled in. People with no signs of the disease what-so-ever, are not filled in at all. Look at the pedigree below and watch how the disease moves through the family members.

Another tool that scientists use is called a Punnett Square. This shows the possible outcomes of a child birth between a male and female. If  you look to the left, you see an example of a Punnett Square. The male is on the top of the square, and has hemophilia because his one "x" chromosome has the disease. (represented by the small "h") The female is completely clean from the disease. All of the people represented inside the box are the possible offspring. In this case, if the couple have a daughter, it will be a carrier no matter what. (represented by one of the "x"'s having the disease) If the couple has a son, he will be completely ok no matter what.

This picture  is another Punnett Square. In this picture the woman is just a carrier and actually doesn't show any signs of the disease. As you can see, the result of this birth could be much worse than before. If this couple had a son, they would have a 50% that their son would have hemophilia. This is a good example how the disease can hide in the generations of people, and show up every once in a while in its full blown form.