How can you work out the chance of a child getting a genetic disease from a family tree?

This is really more of a puzzle than biology. You work this out the same way, no matter how big the tree is. First you need to figure out if the chromosome of the disorder is autosomal or sex-linked. If it mainly affects males, chances are that it is sex-linked. If you aren't completely sure that you've got it right, just run a few examples in the tree and see if it checks out. Next you will have to determine if the gene is dominant or recessive. That tends to be quite easy, because a recessive trait will only surface in a few cases, but always check using examples.Once you know if the disorder is autosomal/sex-linked and recessive/dominant, you have to figure out the genes the parents of the child you're looking at carry. To do that you follow the tree all the way and tracking the genes until the parent you're looking at. The carrier state of other children from the same parents is also important and can tell you a lot about the parents genes. When you know the genes of all the parents you simply draw a square and look at how many combinations result in a child that will inherit the disorder. Each field represents a 25% chance and you just have to add them up.