How these statistical analyses apply to the effect of individual genes on risk can be illustrated by the following considerations. According to the basic rules of genetic inheritance each parent carries two alleles of a given gene, and each offspring inherits one copy from each parent. When one analyzes the alleles of that gene in full sibling pairs, in an average of 25 percent of pairs both siblings will have both inherited the same allele from their mother and the same allele from their father. That means that, with respect to that gene, the siblings will be genetically identical, like identical twins. In an additional 25 percent of sibling pairs, both siblings will have inherited different alleles from both parents and therefore will be genetically unrelated with respect to that gene. In the remaining 50 percent of sibling pairs, both siblings will have inherited the same allele from one parent and different alleles from the other parent, and therefore will share 50 percent of the genetic information for that gene (similar to the average prediction for full sibling or fraternal
