| No. | Mendel's Observations | Deviations |
| 1. | One gene responsible for a single phenotype. | Pleiotropy: Single gene exhibits multiple phenotypic expressions e.g., Disease Phenylketonuria (a single gene codes for enzyme phenylalanine hydroxylase which manifests in form of mental retardation and a reduction in hair and skin pigmentation. |
| 2. | Only one of the parental characters appear in a monohybrid cross (with contrasting character) in the F1 generation. | Incomplete dominance : In a monohybrid cross (with contrasting character) phenotype in the F1 generation did not resemble either of the two parents and was in between the two e.g., Dogflower/ snapdragon/Antirrhinum. |
| 3. | Only one of the parental characters appear in a monohybrid cross (with contrasting characters, in the F1 generation. | Co-dominance : In a monohybrid cross (with contrasting character) phenotype in the F1, generation resembles both the parents e.g., ABO blood group. |
| 4. | All the traits have distinct alternate forms since one gene is responsible for a single phenotype. | Polygenic inheritance : The occurrence of traits are spread across a gradient. Such traits and controlled by three or more genes. e.g, Human height/skin colour. |
