I only know about Phenylketonuria (PKU) because it is an elementary example of an autosomal recessive disease. Newborns are routinely tested, because those with PKU may develop mental retardation on a normal diet. That's about all I knew, but this from Wikipedia:
If PKU is diagnosed early enough, an affected newborn can grow up with normal brain development, but only by eating a special diet low in phenylalanine for the rest of his or her life. This requires severely restricting or eliminating foods high in phenylalanine, such as meat, chicken, fish, nuts, cheese, legumes and other dairy products. Starchy foods such as potatoes, bread, pasta, and corn must be monitored. Infants may still be breastfed to provide all of the benefits of breastmilk, though the quantity must be monitored and supplementation will be required. Many diet foods and diet soft drinks that contain the sweetener aspartame must also be avoided, as aspartame consists of two amino acids: phenylalanine and aspartic acid
This diet has to be followed throughout life, and supplemented with a bitter cocktail of amino acids in the form of a liquid. But perhaps not for much longer, New Options For People With PKU:
For people with the genetic condition known as phenylketonuria (PKU), diet is a constant struggle. They can eat virtually no protein, and instead get their daily dose of this key macronutrient by drinking a bitter-tasting formula of amino acids. Yet drink it they must; deviating from this strict dietary regimen puts them at risk of developing permanent neurological damage. ... In April, a team of University of Wisconsin-Madison researchers will publish the second of two key papers showing that a unique protein derived from whey -- known as glycomacropeptide, or GMP -- is safe for people with PKU to eat. GMP is the first known natural protein that is safe for this group, and these findings are poised to revolutionize the PKU diet. Already, Cambrooke Foods, a Massachusetts company that specializes in the manufacture of medical foods, is in the process of developing GMP-fortified snack foods for commercial sale. ... The first GMP human feeding trial was published in February in the Journal of Inherited Metabolic Disorders. In it, Ney and her team describe the experience of an individual with PKU who volunteered to consume an all-GMP diet for 10 weeks. As the paper explains, not only did the subject enjoy the GMP-fortified snack bar, pudding and sports beverage that supplied most of his daily protein, but the amount of phenylalanine in his blood actually starting going down after he ate these items for a couple of weeks. "And because the subject enjoyed the GMP foods, he was more inclined to eat them throughout the day, which helps keep the body's protein metabolism running efficiently all day long," says Ney. "When he went back to the amino acid formula, he went back to drinking it all in one sitting."
Thank God for modern medicine!