Autosomal recessive disorders
Once again the abnormality is situated on one of the 22 autosomes but in this case the healthy allele takes precedence over the affected one.
The only way the disorder can be passed on is if both parents are carriers and both happen to pass on the recessive gene (just as in our example of a blond child with two dark-haired parents). The risk of passing on an autosomal recessive disorder is therefore 25 percent. If the descendant carries only one mutation he or she will not develop the disorder.
The commonest autosomal recessive disorder is cystic fibrosis. A number of metabolic diseases are also inherited in this way.