Genes come in pairs, with one copy inherited from each parent.
Many genes come in a number of variant forms, known as alleles .
A dominant allele prevails over a normal allele. A recessive gene becomes apparent if its counterpart allele on the other chromosome becomes inactivated or lost. A person who inherits one copy of the recessive allele does not develop disease because the normal allele predominates. However, such a person is a carrier who has a 50-50 chance of passing the altered recessive allele to each of his or her descendants. When both parents are carriers, the chance is one in four that a child will inherit two of the recessive alleles, one from each parent, and develop disease.