Inborn errors of metabolism:
Metabolic diseases due to recessive genes arising from birth are called inborn errors of metabolism.
The term inborn errors of metabolism was coined by Garrod in 1909.
There are three main types of errors of metabolism. They are
I) Phenylketonuria
II) Alkaptonuria and
III) Albinism
I) Phenylketonuria:
Phenylketonuria is a metabolic disease characterized by the accumulation of phenylalanine in the blood.
It is an inborn error of metabolism.
It is a hereditary disease.
It is recessive character caused by recessive genes represented by pp. It is a simple Mendelian character. It is inherited like a typical monohybrid cross.
When these recessive genes are present, the enzyme phenylalanine hydroxylase is not produced.
When this enzyme is absent, phenylalanine cannot be converted into tyrosine
Phenylalanine and its derivatives like phenyl pyruvic acid, phenyl lactic acid, etc. accumulate in the blood and cerebrospinal fluid. The excess of phenylalanine is excreted in the urine.
The children suffering from this disease are known as phenylpyruvic idiots. The disease is characterized by mental defect, feeble mindedness, hypertonicity and light pigmentation.
These disease can be controlled by phenylalanine free diet.
II) Alkaptonuria:
Alkaptonuria is a metabolic disease characterized by the accumulation of alcapton(homogentisic acid) in the blood.
It is an inborn error of metabolism.
It is a hereditary disease.
It is a recessive character caused by recessive genes represented by hh. It is a simple Mendelian character. It is inherited like a typical Mendelian monohybride cross.
When these recessive genes are present, the enzyme homogentisic acid oxidase is not produced.
When this enzyme is absent alcapton(homogentisic acid) a derivative of tyrosine, cannot be converted into acetoacetic acid.
As a result homogentisic acid accumulates in blood. The urine of such persons turns black when exposed to air.
The excess of homogentisic acid is attached to the collagen of cartilage and other connective tissues due to which the ear and sclera are stained black.
III) Albinism:
It is a hereditary defect where the melanin pigments are absent from the skin,hair,eye,etc. Hence affected persons are pale in color and their hairs and eyes are brown in color. Such persons are called albinos. The albinos are highly sensitive to light.
It is an inborn error of metabolism.
It is a recessive hereditary disease caused by recessive genes represented by aa. It follows simple Mendelian inheritance.
When these recessive genes are present, the enzyme tyrosinase cannot be produced.
Hence tyrosine cannot be converted into melanin pigments.
