Phenylketonuria (PKU) is genetic disorder of autosomal recessive gene. PKU patients who are homozygous recessive lack the enzyme phenylalanine hdyroxylase, which converts phenylalanine into tyrosine so, level of phenylalanine increases in the blood and tissue fluids causing muscular and mental problems. Albinsim is also an autosomal recessive mutation. Autosomal dominant gene mutation causes Huntingtons disease, Tay-sachs disease etc. X-linked are sex-linked genes as the genes are located exclusively on X-chromosome. The most common X-linked disease are red-green colour blindness, haemophillia, muscular dystrophy, fragile X-syndrome.