What does it mean when a disease is caused by a recessive gene?
When a trait or disease is recessive, it means that one has to have both alleles (in most cases, one has two copies or alleles of any given gene) being of the disease-type. Usually, one copy comes from the mother and one from the father. Exceptions apply for genes that are sex-linked, mitochondrial, or subject to imprinting (ie Angelman’s or Prader Willi). If one knows that a disease is autosomal recessive (i.e. Mendelian inheritance), and the disease is present in an individual, one can assume that both parents are at the very least carriers because the trait came from both parents. As far as the severity of the disease, this will vary from individual to individual because of variability in the inheritance of other genes +/- environment.
