Inheritance of genetic conditions
A person can be affected by a genetic condition if he or she inherits a particular changed version of a gene. Sometimes the gene can be missing altogether. Different versions of the same gene are called alleles. A genetic condition can be dominant - when the effects of a faulty allele mask those of a working one; or recessive - when the effects of a working allele mask those of a faulty one. A condition may also show X-linked inheritance.