1. Artificial selection

2. For thousands of years, people have tried to ‘improve’ their cattle.

3. Desired features include Docility (making the animal easier to control), Fast growth rates High milk yields. Increases in these characteristics have been achieved by selective breeding.

4. Individuals showing one or more of these desired features to a larger degree than other individuals are chosen for breeding. Some of the alleles conferring these features are passed on to the individuals’ offspring. Again, the ‘best’ animals from this generation are chosen for breeding. Over many generations, alleles conferring the desired characteristics increase in frequency, while those conferring characteristics not desired by the breeder decrease in frequency. In many cases, such ‘disadvantageous’ alleles are lost entirely.

5. CROP IMPROVEMENT Artificial selection and selective breeding. continue to be the main method by which new varieties of crop plants are produced. This has brought about great changes in the cultivated varieties of crop plants, compared with their wild ancestors.

6. EXAMPLES OF CROP IMPROVEMENT 1. The introduction of disease resistance to varieties of wheat and rice

7. Because there is a great loss of yield resulting from infections by fungal diseases, such as head blight, caused by Fusarium, Breeding for resistance to the various fungal diseases can be done by Successful introduction of an allele giving resistance over many generations

8. Rice, Oryza sativa, is also the subject of much selective breeding. The yield of rice can be reduced by bacterial diseases such as bacterial blight, and by a range of fungal diseases including various ‘spots’ and ‘smuts’. The most significant fungal disease is rice blast, caused by the fungus Magnaporthe. Researchers are hoping to use selective breeding to produce varieties of rice that show some resistance to all these diseases.

9. Example 2: The incorporation of mutant alleles for gibberellin synthesis into dwarf varieties so increasing yield by having a greater proportion of energy put into grain

10. Wheat plants now have much shorter stems than they did about 50 years ago. This makes them easier to harvest and means they have higher yields (because they put more energy into making seeds rather than growing tall).

11. Most of the dwarf varieties of wheat carry mutant alleles of two reduced height (Rht) genes. These genes code for DELLA proteins which reduce the effect of gibberellins on growth The mutant alleles cause dwarfism by producing more of, or more active forms of, these transcription inhibitors.

12. Gibberellins are plant growth regulators that are synthesised in most parts of plants. Active gibberellin stimulates cell division and cell elongation in the stem, so causing the plant to grow tall.

13. Example 3 Inbreeding and hybridisation to produce vigorous, uniform varieties of maize

14. Maize, Zea mays, is also known as corn in some parts of the world. It is a sturdy, tall grass with broad, strap shaped leaves. it forms the staple crop in some regions of Africa, and is grown as food for people or animals in Europe, America, Australia, New Zealand, China and Indonesia.

15. To be able to harvest and sell the crop easily, a farmer needs the plants to be uniform. They should all be about the same height and all ripen at about the same time. This can be done by inbreeding and hybridization of the maize plants.

16. Inbreeding is achieved by cross fertilizing two closely related individuals displaying the desired characteristic. Hybridisation is the combining of genes of different varieties or species of organism to produce a hybrid.

17. The inbred maize plants are homozygous are less vigorous the and plants in each generation become progressively smaller and weaker. This inbreeding depression

18. Outbreeding – crossing with other, less closely related plants – produces heterozygous plants that are healthier, grow taller and produce higher yields. However, if outbreeding is done at random, the farmer would end up with a field full of maize in which there was a lot of variation between the individual plants.

19. Therefore, crossing two different inbred varieties produces a hybrid with increased yields. So today, companies produce different strains of maize where each one is the result of a single cross and so all the F1 plants have the same genotype, are uniform and have higher yields.

20. Question 1a) Scientists have found very little evolutionary change in populations of two Australian songbirds, the zebra finch, Taeniopygia guttata castanotis, and the budgerigar, Melopsittacus undulatus. (a) Describe the process of evolution by natural selection. [4] The population of a species, shows genetic variation among organisms; There are selection pressures such as competition, predation; The fitter or better adapted individuals survive, reproduce And pass on these beneficial / advantageous alleles to their offspring. Aand eventually the allele frequency of these desirable traits increases and the undesirable ones reduce. ;

22. (i) Describe the pattern shown by the data in Fig. 2.1. The very large Clutches and very small clutches are very few. 5 is, most frequent clutch size Clutch size shows normal distribution.

23. (ii) The data in this investigation were collected over 60 years ago. The same investigation, carried out today, would produce the same pattern of results. Explain how the selection factors acting on zebra finches would maintain the same pattern of results. This is brought about by stabilising selection ; 4 / 5 clutch size is the mean size so is best selected for and so gives the most surviving offspring ; The low clutch size (2 / 3), gives fewer offspring ; The high clutch size (6 / 7 / 8), not all offspring survive ; because of more competition (between chicks) for, food / parental care ; more predation (of eggs / chicks) and more disease of eggs / chicks;

25. (ii) The Hardy–Weinberg principle cannot be applied to all populations. State two conditions when the Hardy–Weinberg principle cannot be applied. small population ; natural selection ; non-random mating; migration of individuals; new mutations occurring;

26. 2. Maize is an important food crop that has been improved both by selective breeding and by genetic modification. (a)Outline how selective breeding has been used to improve maize. The best / desirable maize plants with desirable characteristics such as more kernels, high yield, fast- growing are crossed ; repeatedly for many generations ; hybridisation has been done by crossing two inbred lines crossed so that F1 hybrids are formed ; This gives more, vigorous and uniform, plants ; Also mutant alleles for gibberellin synthesis have been added to dwarf maize plants so that more yields are achieved ;

27. Name the type of variation shown in Fig. 4.1. Suggest a genetic explanation for this pattern of variation in colour.

28. Type of variation: discontinuous variation; Explanation: Caused by one gene with two alleles ; A dominant and recessive allele ; and it shows a 1:1 ratio of purple to yellow;

30. As, Bt crops / area, increases the number of resistant, pests / species also increases.