Question

How do 'Pleiotropy', 'incomplete dominance', 'co-dominance' and 'polygenic inheritance' deviate from the observation made by Mendel ? Explain with the help of example for each.

Answer 1

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.