WILFWILFTo be able to explain thatTo be able to explain that• Variation between individuals may be either Variation between individuals may be either

continuous or discontinuous and what this continuous or discontinuous and what this meansmeans

• Similarities and differences between Similarities and differences between individuals within a species may be the individuals within a species may be the result of genetic factors, differences in result of genetic factors, differences in environmental factors, or a combination of environmental factors, or a combination of both.both.

Variation

• When organisms reproduce, their offspring When organisms reproduce, their offspring are of the same species and they are all are of the same species and they are all similar to one another and their parents.similar to one another and their parents.

• Variation may be due to:Variation may be due to:– Genetic differencesGenetic differences– EnvironmentEnvironment– BothBoth

• Why does variation occur?Why does variation occur?

• Blending theory……

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

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

Think of some examplesThink of some examples

Shell colouration Shell colouration in the land snail, in the land snail, Cepaea nemoralis Cepaea nemoralis

Tongue Tongue rollingrolling

Hitchhiker’s Hitchhiker’s thumbthumb

Attached or Attached or unattached unattached earlobesearlobes

• There are clear cut differences between some characteristics

• The differences between Mendel’s peas are good examples of clear cut differences

Height in pea plants is an example of Height in pea plants is an example of discontinuous variation – they are discontinuous variation – they are either tall or dwarfeither tall or dwarf

Although there is some variation in both Although there is some variation in both categories, there is no overlap – categories, there is no overlap – individuals are either tall or dwarf and individuals are either tall or dwarf and fall into one category or anotherfall into one category or another

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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.

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

Discontinuous variation shows that:• a single gene gives the characteristic• and the gene is operating with no

environmental effects.

Inheritance and environment

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discontinuous variation

continuous variation

Continuous variation shows that:• either many genes give the characteristic • or the gene or genes are operating with

environmental effects.

Questions

• Page 128 questions 1a & 3

Answers…

1a i) continuous (1)1a i) continuous (1)ii) many genes ii) many genes or or polygenes polygenes or or more than one gene (1)more than one gene (1)3a continuous (1)3a continuous (1)b i) difference in mean of parents b i) difference in mean of parents or or significant significant

difference in length of parents mean cob length of F1 difference in length of parents mean cob length of F1 generation halfway between parental lengths, more generation halfway between parental lengths, more variation in F2 than F1 (2)variation in F2 than F1 (2)

ii) variation present in parental ii) variation present in parental or or F1 generation plants F1 generation plants in parental in parental or or F1 generation genetically identical (2)F1 generation genetically identical (2)

c mean halfway between means for Tom Thumb and F1 c mean halfway between means for Tom Thumb and F1 or or between 9 and 11 cm cob length (1)between 9 and 11 cm cob length (1)

Homework

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

Everything I fully understand Things I still feel unsure about

Information I must remember Ideas to help me remember information