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, ...
