Polygenic Inheritance and Pleiotropy
Polygenic inheritance occurs when one trait is controlled by many genes, while pleiotropy occurs when one gene influences more than one phenotypic effect.
In one line: Polygenic is many genes, one trait; pleiotropy is one gene, many effects.
Concept Practice Coming SoonUnderstand it
- Polygenic inheritance explains traits that show continuous variation rather than clear dominant-recessive categories.
- Each contributing gene may add a small effect, so traits such as human skin colour or height show many gradations.
- Environment can also influence such traits, which is why they are often quantitative.
- Pleiotropy is different: here, a single gene affects multiple traits or body functions.
- For example, a mutation affecting one enzyme pathway can produce several symptoms.
- Exams test this concept by asking students to distinguish many genes affecting one trait from one gene affecting many traits, and by linking examples to the correct term.
Example
Rule to remember
Polygenic traits show additive gene effects and continuous variation.
Pleiotropy follows a single gene change producing multiple outcomes through one biological pathway.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
- 1If a question describes skin colour with several intermediate shades, identify it as polygenic inheritance.
- 2If it describes one gene defect causing intellectual disability, reduced pigmentation, and metabolic disturbance in phenylketonuria, identify pleiotropy.
Method you can reuse
- 1Read the trait description.
- 2Count the direction of genetic effect.
- 3If several genes contribute to one measured trait, write polygenic inheritance.
- 4If one gene produces several effects, write pleiotropy.
- 5Support with a named example.
Visual tip
A distribution curve can help for polygenic traits, with most individuals near the middle and fewer at extremes. A pathway arrow can show one gene affecting multiple outcomes in pleiotropy.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
How is pleiotropy different from polygenic inheritance?
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Common wrong answer
Calling phenylketonuria polygenic just because it has many symptoms is incorrect; the many effects arise from one gene defect.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
Answer plan
- 1
Enough for 1 mark
Polygenic inheritance occurs when one trait is controlled by many genes, while pleiotropy occurs when one gene influences more than one phenotypic effect.
- 2
Add for 2 marks
Example: Human skin colour is commonly explained as polygenic because multiple gene pairs contribute to pigmentation. Phenylketonuria is used as a pleiotropy example because one defective gene affects phenylalanine metabolism and produces multiple effects.
- 3
Add for 3 marks
Polygenic inheritance explains traits that show continuous variation rather than clear dominant-recessive categories.
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