Cancer by Loss of Function of Genes:

Another category is tumor suppressor genes or anti-oncogenes which encodes protein for inhibiting cell proliferation. Inactivation of these genes results excessive proliferation of cells. The protype of this gene is retinoblastoma (Rb) gene which inactivation leads to tumour develop in neural precursor cells of retina. The other gene is Guardian of genome p53 gene which encodes a nuclear phosphoprotein that inhibits formation of small cell lung cancer and colon cancer.

Inborn Error in Metabolism:

A.   E. Garrod (1909) described certain hereditary diseases which lead to certain defects in metabolism. Most of these diseases are due to mutation in genes responsible for production of certain crucial enzymes of metabolic pathways. Inactivation of enzymes leads to accumulation of their substrates which results several abnormalities.

Phenylketoneuria: Phenylalanine metabolism ( phenylalanine hydroxylase )

Alceptoneuria: Phenylalanine and Tyrosine metabolism(homogentisate 1,2-dioxygenase)

Zellweger syndrome: Deficiency or absence of functional peroxisomes