B1.7 Genetic Variation and Its Control
Summary
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.
Genetic material
- A cell is the basic unit of life; all organisms are made up of cells.
- The nucleus is a large organelle found in all cells, that contains the genetic information. (Even red blood cells once had a nucleus).
- Chromosomes are thread-like structures made of DNA found in the nucleus
- Genes are small sections of a chromosome that control the characteristics of an organism.
- These are passed on from parent to offspring, resulting in offspring of plants and animals having similar characteristics to their parents. Eg hair colour and petal colour
- Different genes control the development of different characteristics of an organism.
Sexual reproduction
- Involves two parents.
- They produce male and female sex cells (gametes).
- In humans these are sperm and eggs.
- Fertilisation occurs – the joining (fusion) of male and female gametes.
- The mixture of the genetic information from two parents leads to variety in the offspring.
- Genes are passed on in the gametes, from which the offspring develop.
Asexual reproduction
- Involves only one individual as a parent.
- There is no fusion of gametes.
- There is no mixing of genetic information and so no variation in the offspring.
- These genetically identical individuals are known as clones.
- Examples:
- Bacteria or yeast cells use binary fission
- Plants can use runners, bulbs or vegetative propagation.
- Some invertebrate animals like starfish and hydra can produce asexual offspring.
The causes of variation
- 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.
Genetic factors
- Sexual reproduction – leads to genetic variety in the offspring.
- All offspring (except for identical twins) inherit different characteristics.
- Asexual reproduction – no genetic variation in the offspring (unless mutations occur)
Environmental factors
- Nutrition
- Temperature
- Light
- Physical forces
Cloning
- Humans can carry out various procedures to create clones of plants and animals.
- These new individuals are genetically identical to the parents.
- New plants can be produced quickly and cheaply by taking cuttings from older plants.
Modern cloning techniques include:
Tissue culture
- This uses small groups of cells from part of a plant to grow new plants.
Embryo transplants
- This involves splitting apart cells from a developing animal embryo before they become specialised.
- The identical embryos are then implanted into the wombs of host mothers
Advantages:
- This technique could be used to make many copies of cows that have a high milk yield.
- It would produce a herd of cows much faster than if the original cow was used for breeding in the normal way.
Concerns:
- People do not want this procedure to be used in humans.
- It could be used to provide more embryos for scientific testing.
Adult cell cloning
- The nucleus is removed from an unfertilized egg cell.
- This is replaced with the nucleus of an adult cell (eg skin cell).
- An electric shock is used to start the cell dividing 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.
Advantages:
- This could be used to clone animals with desired characteristics, eg farm animals.
- It could possibly be used to save animals from extinction.
Concerns:
- People do not want human babies to be cloned.
- If this is used in farming, it produces lots of genetically identical individuals;
- If there is a change in the environment, they may all struggle to survive.
Genetic engineering
- Genes from the chromosomes of humans and other organisms can be cut out using enzymes and transferred to cells of other organisms.
Examples of uses in medicine:
- Vaccine production
- Production of insulin for diabetics.
The process:
- Cut out the insulin gene from the DNA of a human cell using an enzyme.
- Remove a ring of DNA from a bacterium and open it up using the same enzyme.
- Insert the insulin gene into the plasmid using another enzyme.
- Enable a bacterium to take up the altered DNA.
- Put the bacterium in a fermenter, and it multiplies many times.
- Each new bacterium contains the insulin gene.
The bacteria produce insulin which can be extracted.
Other uses of genetic engineering
- Genes can also be transferred to the cells of animals or plants at an early stage in their development so that they develop with desired characteristics.
- This could be used to insert ‘healthy’ genes into an embryo that has a genetic disease.
- New genes can also 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.
- GM crops generally show increased yields.
Concerns:
- Long-term, unpredicted effects of consuming GM plants on human health.
- Genes for pesticide resistance may spread from GM plants to their wild relatives, creating pesticide resistant weeds.
- People may want to manipulate the genes of their future children.