Alteration or mutation in a single gene leads to

Mendelian disorders are mainly determined by alteration or mutation in the single gene (monogenic disorders). 

The pattern of inheritance of such Mendelian disorders can be traced in a family by the pedigree analysis. Most common and prevalent Mendelian disorders are haemophilia, cystic fibrosis, sickle cell anaemia, colour blindness, phenylketonuria, thalassemia, etc.