1.7 - Genetic Variation & its control

There are not only differences between different species of plants and animals but also between individuals of the same species. These differences are due partly to the information in the cells they have inherited from their parents and partly to the different environments in which the individuals live and grow. Asexual reproduction can be used to produce individuals that are genetically identical to their parent. Scientists can now add, remove or change genes to produce the plants and animals they want.

Candidates should use their skills, knowledge and understanding to:

■ interpret information about cloning techniques and genetic engineering techniques

■ make informed judgements about the economic, social and ethical issues concerning cloning and genetic engineering, including genetically modified (GM) crops.

B1.7.1 Why organisms are different

a) The information that results in plants and animals having similar characteristics to their parents is carried by genes, which are passed on in the sex cells (gametes) from which the offspring develop.

b) The nucleus of a cell contains chromosomes. Chromosomes carry genes that control the characteristics of the body.

c) Different genes control the development of different characteristics of an organism.

d) Differences in the characteristics of different individuals of the same kind may be due to differences in:

■ the genes they have inherited (genetic causes)

■ the conditions in which they have developed (environmental causes)

■ or a combination of both.

Suggested ideas for practical work to develop skills and understanding include the following:

■ look at variation in leaf length or width, pod length, height. Compare plants growing in different conditions – sun/shade.

B1.7.2 Reproduction

a) There are two forms of reproduction:

■ sexual reproduction – the joining (fusion) of male and female gametes. The mixture of the genetic information from two parents leads to variety in the offspring

■ asexual reproduction – no fusion of gametes and only one individual is needed as the parent. There is no mixing of genetic information and so no genetic variation in the offspring. These genetically identical individuals are known as clones.

b) New plants can be produced quickly and cheaply by taking cuttings from older plants. These new plants are genetically identical to the parent plant.

c) Modern cloning techniques include:

■ tissue culture – using small groups of cells from part of a plant

■ embryo transplants – splitting apart cells from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers

■ adult cell cloning – the nucleus is removed from an unfertilised egg cell. The nucleus from an adult body cell, eg a skin cell, is then inserted into the egg cell. An electric shock then causes the egg cell to begin to divide to form embryo cells. These embryo cells contain the same genetic information as the adult skin cell. When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.

d) In genetic engineering, genes from the chromosomes of humans and other organisms can be ‘cut out using enzymes and transferred to cells of other organisms.

e) Genes can also be transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with desired characteristics:

■ new genes can be transferred to crop plants

■ crops that have had their genes modified in this way are called genetically modified crops (GM crops)

■ examples of genetically modified crops include ones that are resistant to insect attack or to herbicides

■ genetically modified crops generally show increased yields.

f) Concerns about GM crops include the effect on populations of wild flowers and insects, and uncertainty about the effects of eating GM crops on human health.

Suggested ideas for practical work to develop skills and understanding include the following:

■ investigate the optimum conditions for the growth of cuttings, eg Mexican hat plants, spider plants, African violets

■ investigate the best technique for growing new plants from tissue cultures (eg cauliflower).