WILF To be able to explain that

2. WILF To be able to explain that • Variation between individuals may be either continuous or discontinuous and what this means • Similarities and differences between individuals within a species may be the result of genetic factors, differences in environmental factors, or a combination of both.

3. Variation • When organisms reproduce, their offspring are of the same species and they are all similar to one another and their parents. • Variation may be due to: • Genetic differences • Environment • Both • Why does variation occur?

4. Blending theory……

5. Discontinuous Variation  Sometimes the characteristic has just a few discrete categories (like blood group). The frequency histogram has separate bars (or sometimes peaks). .

6. This is discontinuous variation. The characteristics: • have distinct categories into which individuals can be placed • tend to be qualitative, with no overlap between categories • are controlled by one gene, or a small number of genes • are largely unaffected by the environment • Discontinuous characteristics are rare in humans and other animals, but are more common in plants. • Think of some examples

7. Shell colouration in the land snail, Cepaea nemoralis

8. Tongue rolling

9. Hitchhiker’s thumb

10. Attached or unattached earlobes

11. There are clear cut differences between some characteristics • The differences between Mendel’s peas are good examples of clear cut differences

12. Height in pea plants is an example of discontinuous variation – they are either tall or dwarf Although there is some variation in both categories, there is no overlap – individuals are either tall or dwarf and fall into one category or another Number of plants height

13. Continuous variation Much of the variation that occurs within a species is to with height, mass, size or shape. Characteristics that do not fall easily into groups.

14. This variation is usually controlled by several genes, each if which has an effect. It is difficult to measure the individual contribution of each gene, especially as the environment has an influence on this type of variation There are no separate values but a continuous distribution of values

15. Inheritance and environment Discontinuous variation shows that: • a single gene gives the characteristic • and the gene is operating with no environmental effects. discontinuous variation • Continuous variation shows that: • either many genes give the characteristic • or the gene or genes are operating with environmental effects. continuous variation

16. Questions • Page 128 questions 1a & 3

17. Answers… 1a i) continuous (1) ii) many genes or polygenes or more than one gene (1) 3a continuous (1) b i) difference in mean of parents or significant difference in length of parents mean cob length of F1 generation halfway between parental lengths, more variation in F2 than F1 (2) ii) variation present in parental or F1 generation plants in parental or F1 generation genetically identical (2) c mean halfway between means for Tom Thumb and F1 or between 9 and 11 cm cob length (1)

18. Homework • Page 120 – 121 questions 1, 2, 3 & 4