Polygenic inheritance and continuous variation

Overview

Polygenic inheritance, additive effect of polygenes and balanced conclusion about the causes of continuous variation.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is polygenic inheritance?

   Model answer

   The inheritance of a characteristic controlled by many genes, each contributing a small amount to the phenotype.

2. 2

   What are polygenes?

   Model answer

   The many genes of individually small and additive effect that together determine a continuously varying characteristic.

3. 3

   What is meant by the additive effect of polygenes?

   Model answer

   Each contributing allele adds a small increment to the characteristic, so the phenotype depends on how many such alleles the individual carries.

4. 4

   Why does polygenic inheritance produce continuous variation?

   Model answer

   Many genes each of small effect give a very large number of possible totals, so the phenotypes form a continuous range rather than distinct classes.

5. 5

   What shape is the frequency distribution of a polygenic characteristic in a large population?

   Model answer

   A normal distribution, symmetrical about the mean with most individuals near the middle and few at the extremes.

6. 6

   Give three examples of human characteristics showing continuous variation.

   Model answer

   Height, body mass and skin colour.

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