long strings of genes are tightly coiled into packages of DNA called chromosomes . A chromosome is a bit like a book -- a small package containing lots of information. Cells contain 46 chromosomes ; the two chromosomes containing the HD gene are both called number 4. When someone has a child, only one of the chromosomes is passed on. Exclusion testing is done using DNA fingerprinting . It looks at chromosome 4 in all the DNA samples, without looking directly at the HD gene, and finds out whether or not the baby has inherited a copy of chromosome 4 from the grandparent with HD. Based on that, the baby is classified as high risk or low risk of developing HD, and a decision about whether to continue the pregnancy is made. Let's explain it using an example. Alison and Bob want to have kids together but Alison's father, Henry, has Huntington's disease, and Alison doesn't want to have a genetic test for HD. Alison has two copies of chromosome 4: one from her mother Henrietta, and one from her father Henry. The chromosome 4 she inherited from Henry was either the one with the HD mutation, or the normal one -- the chance of each is 50%. The fetus , too, inherits two copies of chromosome 4: one each from Alison and Bob. Neither of Bob's parents had HD, so we don't need to know any more about the chromosome the fetus has inherited from Bob. But the chromosome